http://www.salesforce.com/us/developer/docs/apexcode/index.htm
The 'platform' of the SalesforceDotCom service. Uses a proprietary Java-like language called Apex. Horrible, overengineered, tons of security restrictions, runs nowhere else. Ick.
You are required to use a special version of EclipseIde called the "Force.com IDE" to write it, or an Eclipse plugin (currently at version "20.0") that is buggy with versions of Eclipse newer than 3.6.