This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
In short JOINS join two tables together. For how and what exactly happens this is kind of vendor specific. I mean one can describe some theory but to put into practice you need to write the right JOIN for your Database. What database are you using?