Cycles is nicely integrated into Blender, but can also be build as a standalone renderer . This can be used for testing purposes, but also allows an integration into other 3D programs and pipelines. Note though that the Cycles standalone application is a work in progress.
No precompiled binaries are available at the moment. Instead you can get the source code (see below) and compile your own.
Cycles needs to be started from a terminal: cycles [options] file.xml
Use cycles --help, to get an overview of the available options.
When rendering with the GUI, you can press H to get an overview of available shortcuts. You can pause, cancel or restart a render or try the interactive mode, which allows basic camera navigation.
The Cycles source code can be found here. It's a standalone repository which just contains the Cycles engine. You can find more infos and compile instructions there.
Cycles standalone has a XML API, which is used for mesh data and shader/settings import. There is no real API documentation at the moment, as the API is subject to change, but you can check the source code to get an idea:
app/cycles_xml.cpp: Main XML reading API. xml_read_file() is the main function here, xml_read_scene() calls the different XML loading routines for Geometry, Shaders and Settings then.
render/nodes.cpp: All shader nodes are declared here, check the strings in the add_input() and add_output() calls, these are the names used for shader parameters in XML (such as Scale or Radius for the Subsurface Scattering node).
Please see the current standalone code and its API as a proof of concept only.
We hope to get feedback and/or patches from developers, who are interested in implementing Cycles into other applications. Please get in contact via the bf-cycles mailing list or write a comment on our developer site if you like to get involved: https://developer.blender.org/T38279