The minibuffer has its own set of local keymaps; they contain various completion and exit commands.
minibuffer-local-map is used for ordinary input (no completion).
minibuffer-local-ns-map is similar, except that SPC exits just like RET. This is used mainly for Mocklisp compatibility.
minibuffer-local-completion-map is for permissive completion.
minibuffer-local-must-match-map is for strict completion and for cautious completion.