In Emacs, you can also insert C-h in a key combination. So you can do the basic things like

  C-h k   to pull up the help for a key combination
  C-h f   for a function 
  C-h v   for a variable
  C-h m   for currently active modes

But you can also do things like this:

  C-c C-h   to pull up every key combination that starts with C-c

Emacs has excellent discoverability out of the box thanks to this.