Social networking is the practice of expanding the number of one’s business and/or social contacts by making connections through individuals. While social networking has gone on almost as long as societies themselves have existed, the potential of the Internet to promote such connections is only now being fully recognised and exploited, through Web-based groups established for that purpose.

The Main types of social software used are :

Blogs
The majority are interactive, allowing visitors to leave comments and even message each other via GUI widgets on the blogs.

Instant Messaging
(IM) is a type of online chat which offers real-time text transmission over the Internet.

Forums
An Internet forum, or message board, is an online discussion site where people can hold conversations in the form of posted messages.

Media sharing
Users can upload, share, comment and view videos

Social Networks
Is a platform to build social networks or social relations among people,
Most social network services are web-based and provide means for users to interact over the Internet, such as e-mail and instant messaging.

References:
http://www.webopedia.com/TERM/S/social_software.html
http://en.wikipedia.org/wiki/Blog
http://en.wikipedia.org/wiki/Internet_forum
http://en.wikipedia.org/wiki/Social_networking_service
http://outthinkgroup.com/tips/the-6-types-of-social-media