Reading through this forum I found that many people are struggling with the whole concept of Big-O and Data Structures. While Data structures seems so abstract, they are in practice the ingredient that make or break a program. A thorough understanding of this section is really important.
So I decided to compile this list of material you might find useful. Some of them are not easy to understand but the more you read these things and write programs using them, the better are your chances of understanding them:
From wikipedia (Follow the links on this page. Some of them are very good)