The minibuffer has its own set of local keymaps; they contain various completion and exit commands.
minibuffer-local-mapis used for ordinary input (no completion).
minibuffer-local-ns-mapis similar, except that <SPC> exits just like <RET>.
minibuffer-local-completion-mapis for permissive completion.
minibuffer-local-must-match-mapis for strict completion and for cautious completion.
minibuffer-local-filename-must-match-mapare like the two previous ones, but they are specifically for file name completion. They do not bind <SPC>.