Git will usually open an editor (like vim or emacs) when you run git commit. Pass the -m option to specify a message from the command line:
git commit -m "Commit message here"
Your commit message can go over multiple lines:
git commit -m "Commit 'subject line' message here
More detailed description follows here (after a blank line)."
Alternatively, you can pass in multiple -m arguments:
git commit -m "Commit summary" -m "More detailed description follows here"