KDE Frameworks is a framework of libraries built on the top of Qt. The libraries are divided in four Tiers that define the chain of dependencies.
You don't need to use all the libraries: they are divided in a way that allow you to choose what you really need. For instance if you need to localize your application, use the KI18n library, if you need an archive tool, use KArchive.
You can find the API documentation of the Frameworks on api.kde.org.