Open source is a general term that encompasses many different licenses. They all share some common characteristics, though. These include freely available source code, free redistribution, the license cannot be specific to a product - the code must follow the license regardless of implementation, and the license must not restrict the type of licenses that other products distributed with it must use. A more in-depth definition can be seen at the Open Source Initiative site.
Some of the more common open source licenses include:
Some popular open source software packages include: