Essentially, Genius Hour is an hour in which students explore their passions and are in charge of learning and creating something. It can be extremely open ended or under a specified umbrella topic.
Explore six tips for getting started and links to other worthwhile information about genius hour http://www.edutopia.org/groups/personalized-learning/802456
More genius hour ideas http://www.geniushour.com/2013/03/31/genius-hour-ideas/
Example of a structured topic during Genius Hour(5th Grade): https://www.facebook.com/media/set/?set=a.910158932339097.1073741858.199585953396402&