1 star: I didn't like it at all.
2 stars: I didn't like it that much, but there were a few redeeming qualities.
3 stars: I liked it, but it wasn't phenomenal.
4 stars: I really liked it.
5 stars: I absolutely loved it.
Sometimes, if I'm stuck on what to rate a book, I'll use half stars. For example, if I didn't like a book enough to give it 4 stars, but I liked it more than most books I gave 3 stars to, then I'll give it 3.5 stars.