Eclipse is an integrated development environment (IDE). The Force.com IDE is a plugin for Eclipse which allows it to be used for Salesforce development.
The Force.com IDE is a powerful client application for creating, modifying, testing and deploying Force.com applications. Based on the Eclipse platform, it provides a comfortable environment for programmers familiar with integrated development environments, allowing you to code, compile, test, and deploy all from within the IDE itself.
Useful Documentation
