From OSGeo Wiki
The OSGeo is not a primary source of information regarding Free Software or Open Source. Please refer to the corresponding web sites for a thorough treatment. This page intends to give a quick insight to the topic and explain some basics. Feel free to edit this page if you feel that it is not enough or needs pruning.
Open Source is a public, collaborative development methodology that promotes early publishing and frequent releases. It allows users to study, change, and improve the software, and to redistribute it in modified or unmodified form. Read the Open Source Definition to learn about the details.
Open Source is closely related (and in many contexts perceived as identical) to Free Software.
Commercial Use of Open Source Software
Open Source Software is not an opposite to Commercial Activities! There are many companies and individuals who make a living by producing, maintaining and enhancing Open Source Software or by offering all kinds of services around it. In the geospatial realm it has grown into a healthy Business ecology. Find a company near you that speaks your language using the Service Provider Directory. Add yourself if you provide services.