It's a diary she began shortly before her family went into hiding in a secret annex in an office building/warehouse owned by her father's business partner. It ends somewhat abruptly. We only know what happened because of the testimony of her father, who was sent to a different camp and survived to the end of the war, and that of the people hiding her, who kept the diary safe.

It's pretty much what you'd expect. Anne is a clever teenager with a dramatic streak. She writes about being afraid, but also about being bored, or annoyed with her family or the other people hiding with them. She writes about what she wants to do when the war is over, what kind of person she wants to be when she grows up, and what she misses from before. The tragedy of the diary lies outside its pages.

The diary only covers the time she was in hiding with her family trying not to be caught by the Nazis - it doesn't cover any time in concentration camps. It's been years since I read it but I believe the diary ends when they're captured.

If she kept a diary after getting placed in a concentration camp, it didn't survive.

It's based on her literal diary, from when she was hiding from Nazis.