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.
JRE is java runtime environment. Any computer user can have this installed, you don't need to be a developer to use this. For example if you want to play a game somewhere on the internet written in java, you will need JRE installed.
SDK is the development kit. You need this installed in order to be able to develop java programs.