There are several QGIS mailing lists, IRC channels, forums and user groups you can use to get help from fellow users and developers, or to keep up with current project events. You are welcome to subscribe to any of the lists. Please remember to contribute to the list by answering questions and sharing your experiences. We also provide a website to search the mailing list archives.
The QGIS project provides a gentle GIS introduction, a User Guide, a Coding and Compilation Guide and a API Documentation in english and in some cases also translated into several foreign languages. If you are interested in getting involved with the documentation effort, e.g. translating the english manual into another language, learn how to get started here:
Want to write your own Python plugin? Go here: Developing Python plugins
Want to develop a cpp plugin? Go here: Developing CPP plugins.
A wide range of great plugins exist for QGIS via the QGIS Plugin web portal.
QGIS Core consists on the one hand of the libraries enabling you to build custom tailored application by providing a powerful API. On the other hand it consists of a desktop and server application which expose the possibilities offered by the libraries, offering a user friendly interface.