OpenCV's documentation is online at http://docs.opencv.org/. The documentation includes a combined API reference for OpenCV's current C++ API, its current Python API (which is based on the C++ API), its old C API, and its old Python API (which is based on the C API). When you look up a Python class or function, be sure to read the section about the current Python API (cv2 module), not the old Python API (cv module). The Java API documentation is online at http://docs.opencv.org/java/.
Much of the documentation is also available as downloadable PDF files:
API reference: http://docs.opencv.org/opencv2refman.pdf.
Tutorials: http://docs.opencv.org/opencv_tutorials.pdf. (These tutorials use C++ code. For a Python port of the tutorial's code, refer to Abid Rahman K's repository at http://goo.gl/EPsD1.)
User guide (not a complete guide to all modules but it covers some common points of confusion): http://docs.opencv.org/opencv_user.pdf.
If the documentation seems to leave your question unanswered, try reaching out to the OpenCV community instead. The following sites are good venues for questions, answers, and shared experience:
Official OpenCV forum: http://www.answers.opencv.org/questions/
David Millán Escrivá's blog: http://blog.damiles.com/
Jay Ramhbia's blog: http://jayrambhia.wordpress.com/
OpenCV-Python Tutorials by Alexander Mordvintsev and Abid Rahman K: http://opencv-python-tutroals.readthedocs.org/en/latest/
The support site for my OpenCV books: http://nummist.com/opencv/
Last, if you are an advanced user who wants to try new features, bug fixes, and sample scripts from the latest (unstable) OpenCV source code, have a look at the project's repository at https://github.com/Itseez/opencv/.