Once your game is finished (or mostly so), it's a great idea to create a gameplay video. This lets other people get a quick overview of what your game is like, especially when there are thousands of different games to review.
The quality of video that you make is up to you. Any of these options would be fine:
(easiest) Use your phone or digital camera to record over the shoulder of someone else playing
Video capture software, such as http://fraps.com
For Macs, QuickTime player in OSX Lion can record gameplay. More info here
Anything else you want!
Videos should be uploaded to a site such as http://youtube.com or http://vimeo.com. In the description box: - Be sure to mention which jam site you're with; - Include a link back to your game's page at globalgamejam.org; - Please also add tags #globalgamejam and #ggj12. On your game's page on the GGJ site, you can add a link to the video in the URL box, and it'll show up easily during game searches.
Examples of gameplay videos:
* Audience recording during the game demo, GGJ 2010 Sydney, GNILLEY * Gameplay-only: GGJ 2010 MIT, Quest for Stick * Unity, with script: GGJ 2010, Algoma University Canada, TiltPlain * Over the shoulder: GGJ 2011 Netherlands, GlitchHiker
Pretty much anything is fine, just have fun with it!