FOSS4G stands for Free and Open Source Software for Geospatial. It is the flagship event of OSGeo. Since the first get together, in 2006 in Lausanne, Switzerland, it has been the place where you go for the vibe of the Open Source geoCommunity, where you see the latest developments, find out the latest new super library, or hear of that new innovative product or service. It is the place where you meet the people who wrote the books you read in University, who’s blogs and Twitter accounts you follow. 
The history of FOSS4G is a wonderful story rooted in the beginning of this millennium, within the GRASS and MapServer communities. The underlying idea was to create an event that would offer all geospatial senior or junior professionals, students, professors the possibility to get together and talk about free and open source software for geospatial storage, processing and visualization  In an attempt to answer this quest, it was decided that FOSS4G would find shelter on a different continent each year, so it can come close to all. Do visit the OSGeo dedicated page to shape your idea of what FOSS4G is.