1. Social networking sites: Social networks connect people with all different types of interests, and one area that is expanding in the use of these networks is the corporate environment. Businesses are beginning to use social networks as a means to connecting employees together and helping employees to build profiles

2. Wikis: are websites that allows the visitors to easily add, remove, and edit available content, typically without the need for registration. This ease of interaction and operation makes the wiki an effective tool for mass collaborative authoring.

3. Communication: Web communication protocols are a key element of the Web 2.0 infrastructure. Two major ones are REST and SOAP.

a. REST (Representational State Transfer) indicates a way to access and manipulate data on a server using the HTTP verbs GET, POST, PUT, and DELETE.

b. SOAP involves posting XML messages and requests to a server that may contain quite complex, but pre-defined, instructions for it to follow.

In both cases, access to the service is defined by an API. Often this API is specific to the server, but standard Web service APIs are also widely used (for example, when posting to a blog).

4. Folksonomies: Tags are personalized labels for describing Web content – web pages, blog's, news stories, photos, and the like. Collectively, the set of tags adopted by a community to facilitate the sharing of content is known as a folksonomy.

Web 2.0 services share many attributes. But which create competitive advantage and prompt fast growth? By tracking the services that embrace Web 2.0, we can identify attributes that have made a difference.

The Foundation Attributes that enable the economics of Web 2.0, such as the network effect, the Long Tail and user contributed values, pre-date other attributes by several years and exist in many non-Web 2.0 services They allow services to scale efficiently to accommodate many customers. (e.g., email and bulletin boards).

The Experience Attributes create unique service experiences like decentralization, co-creation, remixabilty and emergent systems that were undeliverable before Web 2.0. Users can tailor services and systems to create new, relevant experiences that meet their needs on their terms.

Earlier users of the phrase "Web 2.0" employed it as a synonym for "Semantic Web," (The Semantic Web is an evolution of the World Wide Web in which information is machine processable (rather than being only human oriented), thus permitting browsers or other software agents to find, share and combine information more easily). and indeed, the two concepts complement each other. The combination of social-networking systems such as a Friend Of Friend (FOF) and XHTML Friends Network (XFN) works to with the development of tag-based, delivered through bogs and Wikis, sets up a basis for a semantic web environment.

I leave you with O reillys definition for As defined by O'Reilly

"Web 2.0 is the business revolution in the computer industry caused by the move to the internet as platform, and an attempt to understand the rules for success on that new platform. Chief among those rules is this: Build applications that harness network effects to get better the more people use them. (This is what I've elsewhere called 'harnessing collective intelligence.')".