This should answer the general question of what it is, if you need more specific please specify.
Java 2 Platform, Enterprise Edition (J2EE) defines the standard for developing component-based multitier enterprise applications.
http://java.sun.com/j2ee/
Java 2 Platform, Enterprise Edition or Java EE (formerly also J2EE) is a programming platform � part of the Java platform � for developing and running distributed multi-tier architecture applications, based largely on modular components running on an application server. The Java EE platform is defined by a specification.
http://en.wikipedia.org/wiki/J2EE
[ January 05, 2006: Message edited by: �dne Brunborg ]