The Eclipse Platform subproject provides the core frameworks and services upon which all plug-in extensions are created. It also provides the runtime in which plug-ins are loaded, integrated, and executed. The primary purpose of the Platform subproject is to enable other tool developers to easily build and deliver integrated tools.
The Eclipse platform itself is a sort of universal tool platform - it is an IDE for anything and nothing in particular. It can deal with any type of resource (Java files, C files, Word files, HTML files, JSP files, etc) in a generic manner but doesn't know how to do anything that is specific to a particular file type. The Eclipse platform, by itself, doesn't provide a great deal of end-user functionality - it is what it enables that is interesting. The real value comes from tool plug-ins for eclipse that "teach" the platform how to work with these different kinds of resources. This pluggable architecture allows a more seamless experience for the end user when moving between different tools than ever before possible.
The Eclipse platform defines a set of frameworks and common services that collectively make up "integration-ware" required to support a comprehensive tool integration platform. These services and frameworks represent the common facilities required by most tool builders including a standard workbench user interface and project model for managing resources, portable native widget and user interface libraries, automatic resource delta management for incremental compilers and builders, language-independent debug infrastructure, and infrastructure for distributed multi-user versioned resource management.
In addition, the Eclipse platform defines a workbench user interface and a set of common domain-independent user interaction paradigms that tool builders plug into to add new capabilities. The platform comes with a set of standard views which can be extended by tool builders. Tool builders can both add new views, and plug new domain-specific capability into existing views.
For a detailed look at the Eclipse platform, see the Eclipse Platform Technical Overview.