For end-users
SNAP and Sentinel Toolboxes provide integrated help directly from the software itself. Additionally, the complete help content is available online.
You can explore the SNAP documentation wiki here and consult the User Guide for detailed usage instructions.
Also, you will find a number of tutorials available from here.
For developers
Developers of SNAP, people interesting in developing new modules or add functionalities to the existing ones should refer to the Wiki for Developer Documentation.