a phrase coined by O'Reilly Media in 2004, refers to a perceived second-generation of Web-based services—such as social networking sites , wikis , communication tools , and folksonomies —that emphasize online collaboration and sharing among users.
Forrester Research, which released a report on Social Computing, says that:
… new social structure is emerging in which technology puts power in communities, not institutions. Forrester calls this evolution Social Computing. Sounds like Web 2.0, right? We think not. And here’s why: Web 2.0 is about specific technologies (blogs, podcasts, wikis, etc) that are relatively easy to adopt and master. Social Computing is about the new relationships and power structures that will result. Think of it another way: Web 2.0 is the building of the Interstate Highway System in the 1950s; Social Computing is everything that resulted next (for better or worse): suburban sprawl, energy dependency, efficient commerce, Americans’ lust for cheap and easy travel.
Collaborative tagging (also know as folksonomy , social classification , social indexing and other names) is the practice and method of collaboratively creating and managing tags to annotate and categorize content . In contrast to traditional subject indexing , metadata is not only generated by experts but also by creators and consumers of the content. Usually, freely choosen keywords are used instead of a controlled vocabulary . 
Social tagging: A study of a web 2.0 service in OPAC
A tag cloud is a set of related tags with corresponding weights. Typical tag clouds have between 30 and 150 tags. The weights are represented using font sizes or other visual clues. Meanwhile, histograms or pie charts are most commonly used to represent approximately a dozen different weights
The website del.icio.us (pronounced as "delicious") is a social bookmarking web service for storing, sharing, and discovering web bookmarks . The site was founded by Joshua Schachter in late 2003 , and was acquired by Yahoo! in 2005 . A non- hierarchical keyword categorization system is used on del.icio.us where users can tag each of their bookmarks with a number of freely chosen keywords (cf. folksonomy ). A combined view of everyone's bookmarks with a given tag is available; for instance, the URL " http:// del.icio.us/tag/wiki " displays all of the most recent links tagged "wiki" ( more about navigating tags ). Its collective nature makes it possible to view bookmarks added by similar-minded users
Technorati is an Internet search engine for searching blogs , competing with Google , Yahoo and IceRocket . As of August 2007, Technorati indexes over 94 million weblogs. The name Technorati is a portmanteau , pointing to the technological version of literati or intellectuals.
Reddit is a social news website where users can post links to content on the web. Other users may then vote the posted links up or down, causing them to appear more or less prominently on the reddit home page.
The site has discussion areas where users may discuss the posted links, and vote for or against others' comments. When there are enough votes against a given comment, it will not be displayed by default, although a reader can display it through a link or preference. Users who submit articles which other users like and subsequently "vote up" receive "karma", points which a user receives as a reward for submitting interesting articles.
Digg is a community-based popularity website with an emphasis on technology and science articles, recently expanding to a broader range of categories such as politics and entertainment. It combines social bookmarking , blogging , and syndication with a form of non-hierarchical, democratic editorial control.
News stories and websites are submitted by users, and then promoted to the front page through a user-based ranking system. This differs from the hierarchical editorial system that many other news sites employ.
An Internet forum is a web application for holding discussions and posting user generated content. Internet forums are also commonly referred to as web forums , message boards , discussion boards , (electronic) discussion groups , discussion forums , bulletin boards , fora (the Latin plural) or simply forums .
Forums allow users to post a "topic" for others to review. Other users can view the topic and post their own comments in a linear fashion, one after the other.
Blogs, short for web logs, are like online journals for a particular person. The owner will post a message periodically allowing others to comment. Topics often include the owner's daily life or views on politics or a particular subject important to them.
Blogs mean many things to different people, ranging from "online journal" to "easily updated personal website". While these definitions are technically correct, they fail to capture the power of blogs as social software. Beyond being a simple homepage or an online diary, some blogs allow comments on the entries, thereby creating a discussion forum. They also have blogrolls (i.e., links to other blogs which the owner reads or admires), and indicate their social relationship to those other bloggers.
In summary, blogs engage readers and build a virtual community around a particular person or interest.
A wiki is a website that allows visitors to add, remove, edit and change content. It also allows for linking among any number of pages. This ease of interaction and operation makes a wiki an effective tool for mass collaborative authoring .
The term wiki can also refer to the collaborative software itself ( wiki engine ) that facilitates the operation of such a site, or to certain specific wiki sites, including the computer science site WikiWikiWeb (the original wiki) and online encyclopedias such as Wikipedia .