What is a grand jury? Here's what to know about the jury that voted to indict Trump.

A New York grand jury voted to indict former President Donald Trump, who has arrived in New York City ahead of his Tuesday arraignment.

The grand jury had been investigating hush money payments to two women who claimed to have had sex with the former president, including adult film star Stormy Daniels.

Trump is expected to be taken into custody as any other defendant would, but as a former president, he’ll have his Secret Service detail beside him and the option to self-surrender. Here’s what to know about the body that indicted him:

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What is a grand jury?

A grand jury is a body of 16-23 people who are tasked with determining whether there is enough evidence to move forward with the case.

The prosecutor is in charge of providing evidence to the grand jury, often in the form of witness testimonies and documents. Both Daniels and Michael Cohen, Trump’s longtime lawyer who aided in the hush money payments, testified before the grand jury.

When a grand jury issues an indictment, it means they believe there is enough evidence to accuse someone of a crime formally. An indictment, at its core, is just a charging document, says Anna Cominsky, an associate professor of law at New York Law School.

"In the first instance, it's filed under seal which means it's not public, which is why none of us know what's on it right now," Cominsky says.

The indictment will likely remain under seal until the arraignment, at which point the public will get a better idea of the criminal charges Trump faces.

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What is the purpose of a grand jury?

The grand jury’s purpose is to decide whether or not the prosecution has presented enough evidence to formally charge someone. They’re not there to decide whether the person has been proven guilty.

"The burden of proof for the prosecutor to obtain an indictment, for the grand jurors to vote an indictment, is significantly lower than that after trial," Cominsky says. "The burden is and will continue to be on the prosecutor to prove whatever case they may have beyond a reasonable doubt, and the indictment is not that proof."

In New York state, a person cannot face a felony trial unless they’ve been indicted by a grand jury.

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Who serves on a grand jury?

In New York state, anyone who is U.S. citizen, at least 18 years old and a resident of the county where they are serving can be a juror. They must also understand and communicate in English and cannot be convicted of a felony.

New York courts have 23 members to a grand jury, with one person acting as a judge-appointed foreperson and one as the assistant foreperson. The foreperson of the grand jury administers the oath to each testifying witness. The grand jury also selects a secretary for record-keeping purposes.

Jurors may serve anywhere from two weeks to over three months depending on the case and county. They’ll hear many cases over this time.

According to the Administrative Office of the U.S. Courts, these three groups are exempt from federal jury service:

• Members of active duty armed forces

• Members of professional fire and police departments

• Active public officers of federal, state and local governments

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What is the difference between a grand jury and a trial jury?

A grand jury determines whether there is probable cause to believe guilt and stand trial, a trial jury (or petit jury) decides whether the defendant has committed the crime as charged. Trial juries consist of 6-12 people and are generally public, while grand jury proceedings are private. This is why the information about Trump's indictment has been an "educated guess," Cominsky says.

"Other than the witnesses themselves saying 'I've gone and spoken with the grand jury,' the grand jury proceedings themselves are secret," she says.

In a grand jury proceeding, defendants and their attorneys do not appear before the grand jury, but in a trial jury defendants can appear, testify and call witnesses on their behalf.

The biggest difference is the outcome – the decision of a grand jury is to indict or not indict, and the decision of a trial jury is the verdict, whether that’s in favor of the plaintiff or defendant in a civil case or a guilty or not guilty decision in a criminal one.

Does a grand jury indictment have to be unanimous?

No – but there must be at least 12 votes toward an indictment in the New York court system. A quorum of 16 grand jurors must be present for evidence and deliberation, and at least 12 grand jurors who have heard the essential evidence must vote.

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This article originally appeared on USA TODAY: What is a grand jury? Here's who voted to indict Donald Trump.