This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
It also depends on the editor that is configured for use with git. When a commit is missing a message, git opens up the configured editor where you are expected to write the commit message and close that editor when done. Take a look at the "core.editor" section here http://git-scm.com/book/en/Customizing-Git-Git-Configuration. I highly recommend reading all those chapters in "Pro Git" book. I've found it extremely useful when I was getting started with Git.
I think Git launched a copy of the vi editor program. Try hitting the "Esc" key, which should force the cursor to the bottom of the screen. Type ":wq" to write and quit. The semi-colon before the "w" is critical.
Note that you can select almost any editor program you want. Most version control systems use whatever the system environment variable named "EDITOR" is set to, and typically default to vi if it isn't set.