Giving a book only one star is never a good thing. When this happens, I didn't enjoy the book point blank and it wasn't a book for me at all. While the book may have had a few strong points, it was definitely not a must read for me.
Two stars is definitely a step up from one star, but still not on my must read list. If I give a book two stars, it's a forgettable book. While I may have enjoyed a few things in this book, it's not a book I'll remember later on.
This book is one of the good ones. If a book gets three stars, I really enjoyed it. This book has a lot (if not all) of the things I look for in a good book. However, the book may only get 3 stars because there were a few things that put me off to it (bad conclusion, annoying character, etc). Overall, though, a good book.
Now here's a great book! Four stars means that the book was simply one of the greats and is definitely on the must read list. There are few things that I dislike in the book and I may feel that the book was missing something (which I often can't put my finger on). However, I'd recommend it in a heartbeat.
Ah, one of the elite books. If a book gets five stars, it's a book I won't be soon to forget. Definitely on the must read list, my only con would be that it ended. A five star book is a book I wasn't able to put down. Trust, a five star rating comes far and few between, but worthy of such.
While I do have the rating system, I highly recommend checking the books out for yourselves. After all, no two bookworms are the same :).