All of Emacs's help is under C-h by default.
C-h C-h (Control-H, pressed twice) brings up the menu, the most useful ones are in bold.
For example, to figure out what a key does, press C-h k (key combination).
|command-apropos. Give a substring, and see a list of commands (functions interactively callable) that contain that substring. See also the apropos command.
|describe-bindings. Display table of all key bindings.
|describe-key-briefly. Type a command key sequence; it prints the function name that sequence runs.
|describe-coding-system. This describes either a specific coding system (if you type its name) or the coding systems currently in use (if you type just RET).
|Show a list of functions, variables, and other items whose
documentation matches the PATTERN (a list of words or a regexp).
|Go to the *Messages* buffer which logs echo-area messages.
|describe-function. Type a function name and get documentation of it.
|Display the frequently asked questions file.
Display information about the GNU project.
|Display the HELLO file which illustrates various scripts.
|info. The info documentation reader. This is where the Emacs manual is.
|describe-input-method. Describe a specific input method (if you type its name) or the current input method (if you type just RET).
|describe-key. Type a command key sequence; it displays the full documentation.
|Show the Emacs manual’s section for the command bound to KEYS.
|view-lossage. Show last 300 characters you typed.
|describe-language-environment. This describes either a specific language environment (if you type its name) or the current language environment (if you type just RET).
|describe-mode. Print documentation of current minor modes, and the current major mode, including their special commands.
|view-emacs-news. Display news of recent Emacs changes.
|finder-by-keyword. Find packages matching a given topic keyword.
|describe-syntax. Display contents of syntax table, plus explanations.
|help-with-tutorial. Select the Emacs learn-by-doing tutorial.
|describe-variable. Type name of a variable; it displays the variable's documentation and value.
|where-is. Type command name; it prints which keystrokes invoke that command.
|Information about Emacs.
|Emacs copying permission (GNU General Public License).
|Instructions for debugging GNU Emacs.
|External packages and information about Emacs.