As humans we all have interests in different subjects and topics. What binds all of us together as a movement or a single organization is that we all share a common idea called freedom in our respective areas of interest. For example, there are people in FSFTN who are software developers, therefore their interest is to build or improve an alternative software for an existing (or) non-existing proprietary software.
Similarly, there are also other kinds of people, who are more interested in bringing an existing software into their regional languages by translating. Other kinds of people are interested in open hardware designs, building decentralized community networks, decentralized local weather stations, improving the open geographical information systems like OpenStreetMap, etc.,
Also beyond these are also people who are interested in legal frameworks, policies of government like Data Protection laws, copyright laws, etc.,
Special Interest Groups are what they represent. (i.e) bringing people of similar interests to act together.