Here are some very useful help functions:
C-h a <keyword>: Apropos. Displays all the commands containing
keyword.C-h b: (describe-bindings) List all key bindings.C-h k: displays what function is bound to the key sequence entered
in the mini-buffer.C-h m: mode informations, describes all the bindings made
available by the major and minor modes.C-h f: displays the description of the function entered in the
minibuffer.C-h v: displays the description and value of a variable.C-h l: (view-lossage) lists the commands that were run. (Since
25.1)C-h C: Coding system for the current buffer.
C-h w command RET: where-is Displays key bindings that calls command.M-x shortdoc-display-group: Display documentation for a group of
functions, e.g. string, buffer, etc. Use TAB to display.