The OpenLayers library supports a wide variety of tile and vector formats including XYZ, WFS, WMS, KML, GML, and GeoJSON. Bing Maps and OSM base maps are supported. Access to the Google Maps API's base maps is available through a third party utility. Utilities for projections and transformations are also provided.
With v3.x, OpenLayers was completely rewritten, and as such the paradigm known from OpenLayers 2 has completely changed. The examples page is useful for beginners, and the API documentation is helpful in finding functions that are not exemplified. Both examples and API are available on the openlayers.org and update with every version.
Use the examples to get a feel for how the API works, and leverage the jsFiddle integration to try out other things.
The API details all functions of the application, but some may be flagged as 'experimental' and may be deprecated or altered in future releases.
For advanced developers, the entire source code is available on GitHub. Due to being open source, the latest version of the API
Source (latest release)
Known issues and logged issues are kept on GitHub.
For information on tagging guidelines for OpenLayers questions see the Meta Q&A at http://meta.gis.stackexchange.com/questions/4425/updating-openlayers-openlayers-2-and-openlayers-3-tags