Hack U is a 24-hour hackathon where teams of up to 4 people build hacks using Yahoo! APIs and open source technologies. Participants brainstorm ideas, code their projects, and present their hacks at the end to win prizes. Yahoo! provides many free and open source projects and APIs that hackers can use to build their hacks, such as YUI, YQL, Flickr, and more. The document gives advice on hacking and demo strategies to help participants develop strong hacks.