If you're new to game jams in general, take a moment to read these basics. They're taken directly from the Global Game Jam website:
The Global Game Jam (GGJ) is the world's largest game jam event taking place around the world at physical locations. Think of it as a hackathon focused on game development. [...]
The structure of a jam is usually that everyone gathers on Friday late afternoon, watches a short video keynote with advice from leading game developers, and then a secret theme is announced. All sites worldwide are then challenged to make games based on that same theme, with games to be completed by Sunday afternoon. [...] The jam is known for helping foster new friendships, increase confidence and opportunities within the community. The jam is always an intellectual challenge. People are invited to explore new technology tools, trying on new roles in development and testing their skills to do something that requires them to design, develop create, test and make a new game in the time span of 48 hours.
The GGJ stimulates collaboration and is not a competition.
Still with us? Then you've got what it takes! This great video is from the 2012 edition, but it explains what the GGJ is about in little more than one minute!
Here's some photos from last year's Global Game Jam in Rome. All photos are from jammer Alessandra Rigano (you can see more in her FB photo gallery).
The first Global Game Jam was held in January 2009, as a project of Susan Gold, in collaboration with Gorm Lai and Ian Schreiber. It had been inspired by great game jams that existed before, such as the Indie Game Jam, Ludum Dare and Nordic Game Jam.
Every year, a global theme is announced for all GGJ venues around the world. Throughout the years, themes have varied greatly: