Run git commit without a message and it'll open up your favorite* text editor to write a commit message. Details on the format of the message will appear in the sample.
First line is the short description. Leave a blank line after it. Everything else is the extended description.
*: for some values of "favorite"; results may vary
Actually, you can do this quite easily from the command line. The trick is to type your message (without a closing quote!) and then hit enter twice, and then type your extended message, close the quote and press enter.