The IntelliJ Platform is a platform for building smart, language-aware IDEs with a comprehensive set of components, including:
- virtual file system
- UI framework (action system, toolwindows, etc.)
- text editor
- lexing, parsing, abstract syntax trees and other language-specific infrastructure
- frameworks for implementing navigation, code completion, inspections, intentions, refactorings, etc.
- version control integration
- debugger framework
- graphical unit test runner
The IntelliJ Platform is open source, and the code is covered by the Apache 2.0 license. This means that you can build both open source and commercial products on top of the platform without paying any royalties to JetBrains.
Please note that the IntelliJ Platform is not a general desktop application framework, and is only designed to be used for IDE-like applications. Basically, if your application requires a code editor, there's a very good chance that the IntelliJ Platform will be suitable as the underlying framework.
For more information about using IntelliJ Platform APIs and building plug-ins for IntelliJ IDEA, please refer to this page.