A command is an InteractiveFunction, that is, a function that can be executed by users via ‘M-x’ or bound to a KeySequence.

When you use ‘M-x’, Emacs prompts you for a command in the minibuffer.

Note: Besides executing a command, you can also interactively execute an arbitrary Emacs Lisp Sexp, using `M-:’ (see EvaluatingExpressions).