Install OpenJDK 6 on Ubuntu 12.04

1. Open up a terminal
2. type this command without quotes "sudo apt-get install openjdk-6-jdk" and hit enter. Just type y if you want to proceed.
3. After it downloaded all the packages needed, then you're ready to go.
4. To check the Java version
1. Just type without the quotes "java -version" 

If you're really wondering what are the differences between Sun's JDK and OpenJDK, here are some useful links:

1. Stackoverflow 
2. Ask Ubuntu 
3. Exploding Java

Google is your friend.