WhereCamp (wherecamp.org) is a recurring geo unconference event. It originally followed on from the massive annual geo conference Where2.0 and originally involved sleeping over (hence "camp"). See 'What is this WhereCamp?' blog
It's an "unconference" because the agenda is agreed on the day in a freeform ad hoc kind of way, with people posting up their ideas and running sessions (talks, discussion, workshops) This works better than you might expect!
Other people adopted the idea and the name, and ran wherecamps elsewhere for people that couldn't make it to the big Where2.0 conference. This includes... WhereCamp.EU - a set of events based in Europe which have quite heavily featured OpenStreetMap, and been organised by some OSMers. See the WhereCamp.EU page for details.