J2SE

J2SE is the Java 2 SDK Standard Edition, available for GNU/Linux (and other OS's) from Sun, for doing desktop Java software development. If you're just learning Java and want to do your development on GNU/Linux, the J2SE (the "Java 2 SDK Standard Edition") is what you want. If you only need to run Java programs, you can just get the JRE.

Note, Sun's J2SE is not Free Software, but is freely downloadable. Portions of it are open source.