As public hearings in the impeachment inquiry enter their second week, here’s everything you need to know about allegations against President Donald Trump, his defense, and why the phrase “quid pro quo” is dominating the debate:

So what happened exactly?

It started with a single whistleblower, who last August warned that in a July 25 phone call with Ukraine’s president, Volodymyr Zelenskiy, Trump pressured the new government to launch a politically charged investigation.

Trump wanted two things: First, he wanted Ukraine to prove a theory that its own corrupt politicians in 2016 tried to sway American voters in favor of Democrat Hillary Clinton. While some Ukrainian politicians did support Clinton over Trump in largely public ways, U.S. intelligence and a bipartisan Senate inquiry found that Russian operatives — at the behest of their government — engaged in a far more widespread, invasive and secretive campaign to sway voters in support of Trump.

Second, Trump wanted Ukraine to investigate one of his 2020 political rivals, former Vice President Joe Biden, and his son Hunter for his work on the board of Ukrainian energy company Burisma.

The whistleblower described his colleagues on the call as “deeply disturbed” by Trump’s demands and detailed an internal effort internally to “lock down” all records of the discussion.

Since that bombshell allegation was made public, other witnesses have corroborated the person’s account and the White House released a summary of the call that describes Trump asking Ukraine for a “favor” and to investigate the Bidens.

Andrew Harnik/AP

So what is the ‘quid pro quo’? Is it bad?

“Quid pro quo” is Latin meaning “something for something.” The term is used in employment law, for example, to prohibit sexual harassment, and under federal laws against bribery.

When it comes to foreign policy though, such negotiations are actually quite common when the U.S. demand is tied to America’s national security interests. One example is asking a country to tackle corruption before receiving U.S. assistance.

In the case of Ukraine, however, the alleged quid pro quo wasn’t about national security but about dangling U.S. money and a presidential summit at the White House in exchange for investigating a political rival.

By the time Trump had asked Ukraine’s president for a “favor” on the phone on July 25, he had already suspended nearly $400 million in U.S. military aid. Advisers to Trump wanted the aid released because it was needed by Ukraine to fend off Russian aggression. They also wanted a White House summit between the two leaders to show U.S. support for the new government.

As one Army officer assigned to the National Security Council – Lt. Col. Alexander Vindman – testified, Trump was pressuring a foreign power that desperately needed America’s help in exchange for its willingness to target U.S. citizens for the president’s personal gain.

“There was no doubt” about what Trump wanted, Vindman told Congress.

Trump has denied anything was wrong with the phone call, insisting it was “perfect.” Some Democrats, though, compare it to extortion.

OK, but is that an impeachable offense? Is it illegal?

The short answer is that it doesn’t matter if the act is illegal when it comes to impeachment.

Impeachment is a political process enshrined in the Constitution. It essentially provides a pathway for elected lawmakers to remove presidents whose actions are considered such a gross abuse of power that elected officials don’t want them to finish out their term.

And while the Constitution references “high crimes and misdemeanors,” the founding fathers didn’t necessarily interpret that to mean a violation of U.S. law. As Benjamin Franklin, in a 1787 speech put it: Without impeachment, the only “recourse” for the American public in dealing with a leader who had “rendered himself obnoxious” was assassination. And allowing for impeachment under the Constitution would give America a way to examine misconduct without resorting to violence.

On the question of whether Trump broke any laws, political campaigns are prohibited from taking anything of value from a foreign national. But the Justice Department declined to pursue the case because it didn’t have evidence that campaign finance laws were violated; officials told reporters it would have been difficult to “quantify” what Trump would have gotten from Ukraine in return.

As for facing criminal prosecution, the Constitution doesn’t say whether a sitting president can be charged in office, and the Justice Department has long had a policy that it shouldn’t happen because it would undermine the executive branch.

What’s Trump’s defense?

Alex Wroblewski/Getty Images

Trump and GOP lawmakers insist there never was a quid pro quo with Ukraine and that the original whistleblower was motivated by politics. They note that Ukraine eventually received the money and claim that the government there wasn’t even aware of the hold on aid.

These claims though run counter to the testimony of several White House aides and top diplomats.

It is correct that Ukraine eventually received the military aid with the hold released on Sept. 11. Republicans tie the decision to a Sept. 1 meeting between Vice President Mike Pence and Ukrainian officials in Warsaw in which Ukraine’s anti-corruption agenda was discussed. But there also was mounting pressure from the Pentagon, State Department and Congress. Democratic Rep. Adam Schiff of California has suggested the aid was released for another reason — the White House learned that a whistleblower complaint had been described to Congress on Sept. 9 — 48 hours before the hold was lifted.

The whistleblower’s claims also have been substantiated by numerous other witnesses, including several who will testify this week.

But another key argument by Republicans could stick. They note that Democrats have been talking about impeachment since Trump took power in 2017. GOP lawmakers say the latest inquiry is just the latest – and most coordinated effort – to overturn the results of the 2016 election.

And as time inches closer to the 2020 election, it will become harder for Democrats to argue that they’ll need to act, rather than let voters decide at the polls.

But what about Biden? Isn’t it fair for Trump to investigate?

At the center of Trump’s allegation is a widely debunked theory that Biden – as vice president under President Barack Obama – delivered his own quid pro quo to Ukraine to help his son, Hunter, who was serving on the board of a Ukraine gas company, Burisma.

It’s correct that Biden in 2016 pressured Ukraine to fire the prosecutor general, Viktor Shokin, and threatened to withhold nearly $1 billion in U.S. aid if Shokin wasn’t removed. But according to testimony by U.S. envoy Kurt Volker and others, Biden’s position was in line with U.S. policy and backed by other international powers that agreed Shokin was ignoring corruption and needed to go.

Volker also testified that it was “quite possible” that Ukrainian gas executives believed they were “buying influence” through Biden’s son by putting him on the board of Burisma. The vice president’s son was paid to serve on the board (at one point, his firm collected $166,666 each month from Burisma as both Hunter Biden and his associate sat on the board), and State Department official George Kent testified he warned about a perceived conflict of interest.

But no evidence has surfaced that Joe Biden acted to help his son, nor that his Hunter Biden violated any laws, and the Bidens deny any wrongdoing.

So what happens next? Who is testifying?

Andrew Harnik/AP

After weeks of closed-door depositions, House Democrats last week moved on to open public hearings (here is a list of who testified and what they said).

Last week, the House Intelligence Committee heard from two career diplomats – Bill Taylor and George Kent – who testified to their concerns about the alleged pressure campaign and its impact on U.S. security interests. Marie Yovanovitch, the former ambassador to Ukraine, testified separately to being the subject of a smear campaign by Trump’s lawyer, Rudy Giuliani, before being recalled suddenly from her post.

This week Vindman, the Army officer detailed to the White House, will testify, as well as U.S. envoy Volker and other staffers. Wednesday could be the most consequential hearing yet — Gordon Sondland, the Trump megadonor appointed as U.S. ambassador to the European Union is believed to have a direct line to Trump on Ukraine and may be able to speak to exactly what the president ordered.

After the House Intelligence Committee moves through its witnesses, the inquiry will shift to the House Judiciary Committee, which would draft articles of impeachment and vote on them, followed by a full vote in the House, which requires a simple majority.

If Trump is impeached, does that mean he leaves office? What happens next?

Impeachment doesn’t force presidents out of office. It’s more like a political black eye delivered by the House that kicks off a “trial” in the Senate.

If that happens, a group of House lawmakers — all Democrats – would essentially act as prosecutors and argue the case before the Senate. Trump and his legal team would have equal time to put on their defense, with the Senate acting as a kind of jury.

If at least two-thirds of the nation’s senators vote against Trump, he would be removed from office – a high bar in a Senate currently controlled by Republicans. (The Senate doesn’t have the power to send a president to jail, only oust him.)

One big concern for Democrats – and a reason House Speaker Nancy Pelosi seemed to be initially reluctant to consider impeachment – is that Trump could be impeached by the House, only to be acquitted by the GOP-controlled Senate and possibly come back stronger politically as his base rallies to his defense.

That’s what happened in 1998 when Democrat Bill Clinton was impeached for lying under oath to try to hide an extra-marital affair with White House intern Monica Lewinsky. While the GOP had clear evidence of the wrongdoing, he was acquitted by the Democratic-controlled Senate and saw a boost in his approval ratings.

(President Andrew Johnson also was impeached and acquitted by the Senate in 1868).

To hear Democrats tells it, nothing less than the soul of the country is at stake.

“There comes a time when you have to be moved by the spirit of history to take action to protect and preserve the integrity of our nation. I believe, I truly believe, the time to begin impeachment proceedings against this president has come,” said Rep. John Lewis., D-Georgia, on the House floor last September. “To delay or to do otherwise would betray the foundation of our democracy.”

Republicans like Texas Rep. Mac Thornberry disagree.

“I believe it was inappropriate. I do not believe it was impeachable,” he recently told ABC’s “This Week.”

As for Trump, he says it’s nothing more than “the Greatest Witch Hunt in American History!”

ABC News’ Trish Turner and Mike Levine contributed to this report.

Editor’s Note: This version corrects a reference to Clinton to note he had a boost in approval rating after impeachment, but was not up for reelection.