From Google definition: Social software lets people rendezvous, connect or collaborate by use of a computer network. The term came into more common usage in 2002, largely credited to Clay Shirky who organized a "Social Software Summit" in November of that year.

From Wikipedia:

The term Social software is normally applied to range of web-enabled software programs. The programs usually allow users to interact, share, and meet other users. This computer-mediated communication has become very popular with sites like MySpace and youtube and has resulted in large user bases and billion dollar purchases of the software and their communities by large corporations (Fox purchased myspace and google purchased youtube).

The more specific term collaborative software applies to cooperative information sharing systems, and is usually narrowly applied to the software that enables collaborative work functions. Distinctions between usage of the terms "social" and "collaborative" are in the applications or uses, not the tools themselves, although there are some tools that are only rarely used for work collaboration.

Examples of Social Software

Instant messaging, chat, online forums, blogs, wikis, collborative real-time editor and more. For complete rendering of examples, see http://en.wikipedia.org/wiki/Social_software .