Open Source Software is software for which the source code is available for free. Usually the source code and compiled versions can be used/changed/redistributed with simple restrictions as detailed in the accompanying license. Even though the source code is readily available, some companies like Red Hat charge for support with their product. The Open Source principles can also be applied to concepts besides software.
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: