From the command line you can run:
For most commands you will find a man page, also you can find man pages for different topics, if you want to search you can run:
it will show you all the man pages that have the string mytopic in the name or description
You can consult Linux manpages for that. To invoke it for a particular command use man <command>
The man man command will provide you with more information on how to browse the documentation.