Ross Mayfield writes:
Social Software, a term coined by Clay Shirky, is the design of systems that supports groups with an underlying value proposition of building social capital
Social Software is not that new, but its currently a growing and evolving sector marked by a high level of cross-polinization. The level of innovation defys easy categorization.
Properties include people-centricity, low communication costs, low transaction costs that encourage adoption, easy group forming, triads rather than pairs, treating groups as first class objects in the system and adapting to the social network (heterarchy) rather than requiring it to adapt it it (heirarchy). Second order effects include emergence, reputation, different values at different scales, transparency, decentralization and fun parties.
Other dimensions to view this space include enterprise vs. consumer, how connections are formed, different values at different scales, what markets are cannibalized, what cultures (not markets, but dont reach for your gun) are served and open vs. closed.
These dimension easily blur. Take for example the distinction of enterprise vs. consumer. Social Software adoption is being driven in the enterprise from the bottom up. Initially, it users as developers bringing in their own tools like personal publishing and wikis plus (shameless plug here) enlightened companies serving both users and enterprises at different scales.