Basic premise is that a little girl (Lyra) is in possession of the Althimeter (?sp), which when a question is asked can give answers (if you know how to read it). However to complicate matters there are 'sides' looking to gain possession of the 'Golden Compass'.
This is a very very basic premise, there is a lot more to the books then just this. I might get slated here, but I found it very similar to Dune. Its one of those books that you can't really describe, it has to be read.