The Eclipse project is composed of three subprojects, Platform, Java development tools (JDT), and Plug-in development environment (PDE). The Eclipse Platform is an open extensible IDE for anything and yet nothing in particular. It allows tool builders to independently develop tools that integrate with other people's tools so seamlessly you can't tell where one tool ends and another starts. The JDT project provides the tool plug-ins that implement a full-featured Java IDE supporting the development of any Java application, providing refactoring support, incremental compilation, smart editing etc. The JDT project allows Eclipse to be a development environment for itself. The PDE project provides the mechanisms that enable the community to build plug-ins for eclipse.
The Eclipse SDK includes the Eclipse Platform, Java development tools, and Plug-in Development Environment, including source and both user and programmer documentation.