Courtney was 14, and still had braces, when Jeffrey Epstein asked her to give him a massage.

More than a decade ago, multimillionaire Jeffrey Epstein received one of the most lenient sentences for serial sex offences in US history.

In 2008, Epstein, 66, pleaded guilty to two state prostitution charges involving a 17-year-old girl, and he served 13 months in Florida’s Palm Beach County Jail.

Unlike other prisoners, including the majority of sex offenders, Epstein was housed in a private wing of the jail… but he didn’t spend too much time there. He was able to hire his own security detail and was allowed “work release” for up to 12 hours a day, meaning he was able to leave to work at his downtown office.

After 13 months, Epstein was released for a year of probation. Despite probation, he was allowed to take many trips on his private jet to his homes in New York and the U.S Virgin Islands.

All of this is due to a very lenient deal handed to Epstein and his lawyers by prosecutors. At the time, the deal was sealed, meaning not even his victims knew what it contained.

Fast forward 11 years to last week, when a federal appeals court in New York ordered the unsealing of up to 2000 pages of documents that are expected to show evidence relating to whether Epstein and his partner, Ghislaine Maxwell, were recruiting underage girls and young women as part of an international sex trafficking operation. Maxwell, 57, has never been charged.

Headlines have mentioned raids, a "treasure trove" of photos, and various famous names - including Prince Andrew.

Here's everything you need to know about the case:

Who is Jeffrey Epstein?

Jeffrey Epstein made his millions running a money management firm catering to super-rich clients, such as Victoria's Secret founder Les Wexner.

Since being released on probation, Epstein's business dealings have been kept very private.

What we do know is that he has millions wrapped up in his various properties, including his New York City mansion worth US$77 million (AU$110m).

He also owns property in New Mexico, Paris, Florida and on the U.S Virgin Islands, where he has a private island.

In short: He's very rich and has friends in high places.

What are the charges he is facing now?

Epstein was arrested on Saturday night at Teterboro Airport in New Jersey, where he had returned on his private plane from Paris.

He has pleaded not guilty to sex trafficking charges as he was accused of luring dozens of girls to his luxury homes and paying them for sex acts.

An indictment unsealed in federal court in Manhattan accuses the 66-year-old of arranging for girls as young as 14 to perform nude "massages" and other sex acts, and paying some girls to recruit others, from at least 2002 to 2005.

According to investigators he sexually abused nearly three dozen girls, mostly 13-16 years old, at his Palm Beach mansion from 1999 to 2006.

The former hedge fund manager "intentionally sought out minors and knew that many of his victims were in fact under the age of 18, including because, in some instances, minor victims expressly told him their age", the indictment said.

Epstein has said in earlier court filings his encounters with alleged victims were consensual and he believed they were 18 when they occurred.

Federal agents allegedly found "a vast trove of lewd photographs" of young-looking women or girls locked in a safe in his NYC mansion.

Wearing dark blue jail scrubs, Epstein entered his plea before US Magistrate Judge Henry Pitman in Manhattan on Monday to one count of sex trafficking and one count of sex trafficking conspiracy.

What do Epstein's accusers say?

Some of Epstein's accusers have criticised his first sentencing as too lenient.

Over the years, a number of alleged victims have come forward - most notably Virginia Giuffre, formally known as Virginia Roberts, who said in a sworn affidavit that Maxwell recruited her to be a "sex slave" with she was just 16.

After his arrest, she said she was "deeply pleased", CNBC reported.

"I am deeply pleased that federal prosecutors in New York have arrested Jeffrey Epstein and are on the case in a serious way. It is time for Jeffrey Epstein and those who participated and enabled his sex crimes to be brought to true justice."

Courtney Wild accused Epstein of forcing her into sex acts in the 2008 case. She said she was a 14-year-old high school student at the time, a young teen still in braces.

Wild, now in her 30s, said she was lured to Epstein's house: "I remember standing in his kitchen, and he also had a lot of girls there at the time," she recalled to ABC News on Monday.

She said Epstein convinced her to recruit other girls and that he told her "he wanted them as young as I could find them".

Michelle Licata, another accuser, told ABC that she was recruited by a high-school friend when she was 16 to give Epstein massages.

During an interview with authorities, part of which ABC aired, Licate recalled Epstein told her how beautiful she was and it made her uncomfortable: "Then he wanted me to rub his back," she said. "And then he kept asking me to go lower and lower."

Accuser Sarah Ransome said in a statement that his recent arrest was "a step in the right direction" for holding him accountable.

Speaking to a press conference about the new charges, US Attorney Geoffrey Berman said Epstein's alleged victims deserved their day in court.

"The alleged behaviour shocks the conscience, and while the charged conduct is from a number of years ago, it is still profoundly important to the many alleged victims, now young women," he said.

Berman said a search of Epstein's Manhattan mansion uncovered evidence including nude photographs of what appeared to be underage girls.

He encouraged other victims to come forward and contact prosecutors.

How are Donald Trump and Prince Andrew involved?

Known for socialising with politicians and royalty, Epstein once had friends including US President Donald Trump and former president Bill Clinton, and according to court papers, Britain's Prince Andrew.

None of those people were mentioned in the indictment.

Epstein's confidante and alleged "pimp" Ghislaine Maxwell has been photographed alongside many powerful people who are also linked to Epstein including Trump, Clinton and Prince Andrew. She was even a guest at Chelsea Clinton's 2010 wedding.

In a tweet on Monday, former President Clinton’s spokesperson Angel Urena said the former president "knows nothing about the terrible crimes Jeffrey Epstein pleaded guilty to in Florida some years ago, or those in which he has been recently charged in New York".

According to the Miami Herald, alleged victims Virginia Giuffre worked at Trump’s Mar-a-Lago club when she was introduced to, and began to be sexually abused by, Epstein.

Giuffre alleged in court documents Epstein forced her to have sex with Prince Andrew, the Queen's third child and 8th in line to the British throne, in a number of places including New York, London and a private island.

When the allegations came out in 2015, Buckingham Palace issued three separate statements saying the story was false.

"It is emphatically denied that the Duke of York had any form of sexual contact or relationship with Virginia Roberts. Any claim to the contrary is false and without foundation," one read.

A photo, showing Prince Andrew with his hand around Giuffre's waist does show that they met. In the background of the photo, Maxwell is shown smiling at the camera.

Trump, now President of the United States, considered Epstein a close friend and praised him in an interview with New York magazine in 2002.

"I've known Jeff for fifteen years. Terrific guy," Trump said.

"He's a lot of fun to be with. It is even said that he likes beautiful women as much as I do, and many of them are on the younger side. No doubt about it - Jeffrey enjoys his social life."

Last November, the Miami Herald published a series of stories, titled Perversion of Justice, that described the ways in which Florida's US attorney, Alexander Acosta, worked in conjunction with Epstein's lawyers to engineer the non-prosecution agreement - and keep it secret from Epstein's victims. Acosta is now President Donald Trump's secretary of labour.

What will happen to Jeffrey Epstein next?

In court on Monday, prosecutors argued Epstein was a major flight risk, considering his wealth and access to private jets, numerous other residences and private island.

Epstein was ordered to be held without bail until at least July 15 pending a detention hearing, where prosecutors are expected to continue their argument that he should be detained without bail pending a trial.

If convicted, he will face up to 45 years in prison.

-With AAP.