There's a search function on the forum and a wealth of information if you use it.
Normalisation, or Normalization if you're American

, is basically the process of breaking down all the elements of your database into the best possible design. It involves removing such design follies as non-atomic fields, repeating groups, irregular key dependancies, and calculated fields (non-key dependancies).
Generally, if your database reaches Third Normal Form (3NF) then it's a fair bet that it is normalised. There are forms above and beyond (4th, Boyce-Codd, 5th) but generally, in most projects, people stop at 3NF. Usually, if it's "in 3NF" then it's good enough for the higher forms.
First of all, you have to get your database to First Normal Form (1NF). This, as I've written on
this FAQ involves ensuring atomic fields and removing repeating groups. If you don't achieve 1NF then you
can't achieve Second Normal Form (2NF), removing fields not dependant on the table's key to their own table. And, if you can't achieve 2NF, then you can't achieve 3NF, removing calculated fields.
Unfortunately, I haven't got around to writing an FAQ for 2NF and 3NF as I'm writing stuff for my own site just now, but I'll eventually get around to it.
Open
Google and type in Normalisation or Normal Form also. There's literally thousands of sites out there with explanations and/or examples.