Katniss Everdeen lives in post-apocalyptic Panem, where the U.S once used to be. Panem used to be divided into 13 districts and the Capitol, but after a rebellion, District 13 was destroyed. To keep the various districts in check, the Capitol demands that every year each district send two teen tributes, one male and one female, to compete in the televised Hunger Games, a fight to the death where there can be only one victor. However, Katniss thwarted the Capitol in the last Hunger Games, and both the tributes from District 12 survived. She pretended to be madly in love with her fellow tribute Peeta, and would have poisoned them both, forcing them to let both survive rather than have no winner.
Katniss does not realize just how much her defiant act has inspired people in the various districts. There is discord and discontent, and the President of Panem threatens to kill not only Katniss, but Peeta and everyone Katniss ever loved if she doesn't help him improve the image of the Capitol and the power of the government. Katniss has to continue her act as Peeta's beloved, made harder by the fact that Peeta genuinely loves her, and she prefers Gale, a miner in District 12. When travelling around Panem on their victory tour, Peeta and Katniss play at being passionately in love, and Peeta even proposes, but this does not stop rebellions in several of the Districts.
Every 25 years, there is a Quarter Quell as well as the regular annual Hunger Games. This year, it seems the contestants of the Quarter Quell will be selected from previous winners. Katniss is the only female contestant ever to survive the Games from District 12, and therefore has to go back and fight for her life once more, this time against people who have already proven their battle prowess by killing all their opponents. Peeta insists on being the male tribute, and this time, hoping that the Capitol and the President might spare her family if she dies, Katniss is determined that Peeta will be the one to survive.
Catching Fire is a very dark book, and the world that Suzanne Collins has created is a very bleak one. The book is set in a dystopian and totalitarian future, where the government forces its subjects to volunteer and send their young to get killed, in order to keep them subjugated and too scared to rebel. In the Capitol, the Hunger Games are prime time entertainment, it's clear that the people who live there have no idea of the suffering of the people in the various districts.
Katniss realizes that unwittingly, she has become a symbol for the rebellion against the Capitol. She thought that by surviving the Hunger Games, she could live out the rest of her life in relative comfort with her family, and that outside the occasional television interview, she could forget about her time in the Games, and her pretend romance with Peeta. Instead, she discovers that the Capitol have been keeping an extra close eye on her, and her one kiss in the woods with Gale has pretty much ensured that he will be the first to die if things don't go the way the Capitol wishes. Also, while she's unable to return Peeta's romantic feelings, she values him as a friend and their shared experience in the Games means that only he can understand how she feels on occasion.
Both The Hunger Games and Catching Fire are very well-written, gripping reads, if dark and a bit depressing. Having heard rumours that the third and final book in the trilogy, Mockingjay, which was released at the beginning of September is the most depressing of the three, I'm holding off on reading it for a bit, as I don't think I'm up for it at the moment.