David Petraeus: Retired four-star Army general who became the director of the CIA in September 2011 after a distinguished military career, including commander of US and international troops in Iraq and, later, Afghanistan. He resigned from his post at the CIA last Friday after admitting to showing "extremely poor judgment by engaging in an extramarital affair."
Paula Broadwell: Author of All In: The Education of General David Petraeus, the recent hagiographic biography about General Petraeus.
Like Petraeus, she graduated from West Point, America's elite Army college. She met Petraeus in 2006 at Harvard, where she was a graduate student and she decided to write a Ph.D. dissertation on him once he took command in Afghanistan in the summer of 2010. She visited Petraeus in Afghanistan, joined his inner circle and began an affair with him in late 2011, two months after Petraeus was named CIA director by President Obama.
Jill Kelley: A Florida woman who became friendly with Petraeus and his wife, Holly, when the general was head of Central Command in Florida, just prior to his posting in Afghanistan. Kelley volunteers to help injured veterans and their families at the base in Tampa and is also an unofficial social coordinator for senior military leaders. Kelly began receiving harassing anonymous emails in May 2012, accusing her of inappropriate flirting with Petraeus. Kelley reported the emails to the FBI. There is no evidence she had an inappropriate relationship with Petraeus.
Unnamed FBI Agent: A friend of Kelley who referred her anonymous emails to the FBI's cyber crimes unit for investigation. The FBI traced the emails to Broadwell and during their investigation of her, also turned up the sexually explicit emails between Broadwell and Petraeus. The unnamed FBI agent was removed from the case this summer after his superiors became concerned he was obsessed with the case.
Evidence of his obsession: he sent shirtless photos of himself to Kelley [a married woman] at some point before the investigation began, something the FBI learned only later.
John Allen: Current commander of US and NATO troops in Afghanistan who is under investigation by the Pentagon after the FBI probe into Petraeus uncovered thousands of pages of email transcripts containing "potentially inappropriate" communications between General Allen and Jill Kelley. Allen has close ties to Petraeus, serving as his deputy at Central Command and succeeding him in Afghanistan. The precise nature of the emails between Allen and Kelley and the details of their relationship are unknown at this point.