Web 2.0 refers to a perceived second generation of web-based communities and hosted services — such as social-networking sites , wikis , and folksonomies — which aim to facilitate creativity , collaboration, and sharing between users.
"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." 
Web 2.0 is all about collabaration and conversations with communities of people with something in common who interact with each other.
What is Web 2.O?
What is Web 2.O? “ Web 1.0” “ Web 2.0” Read-Only Web Read-Write Web
Micro-blogging is a form of blogging that allows users to write brief text updates (usually less than 200 characters ) and publish them, either to be viewed by anyone or by a restricted group which can be chosen by the user. These messages can be submitted by a variety of means, including text messaging , instant messaging , email , MP3 or the web .
The most popular service is called Twitter , which was launched in July 2006 and won the Web Award in the blog category at the 2007 South by Southwest Conference in Austin, Texas .  The main competitor to Twitter is Jaiku .
The popular social networking websites Facebook and MySpace also have a micro-blogging feature, called "status update".
Micro-blogging is about “what are you doing right now.”
A social network service focuses on the building and verifying of online social networks for communities of people who share interests and activities, or who are interested in exploring the interests and activities of others, and which necessitates the use of software .
Most social network services are primarily web based and provide a collection of various ways for users to interact, such as chat , messaging , email , video , voice chat , file sharing , blogging , discussion groups , and so on.
Social Networking Sites http://en.wikipedia.org/wiki/Social_network_service
A podcast is a digital media file, or a related collection of such files, which is distributed over the Internet using syndication feeds for playback on portable media players and personal computers .
Though podcasters' web sites may also offer direct download or streaming of their content, a podcast is distinguished from other digital media formats by its ability to be syndicated, subscribed to, and downloaded automatically when new content is added, using an aggregator or feed reader capable of reading feed formats such as RSS or Atom .
A wiki is a type of computer software that allows users to easily create, edit and link web pages.
Wikis are often used to create collaborative websites , power community websites, and are increasingly being installed by businesses to provide affordable and effective Intranets or for use in Knowledge Management .
Software as a service ( SaaS ) is a software application delivery model where a software vendor develops a web-native software application and hosts and operates (either independently or through a third-party) the application for use by its customers over the Internet.
Customers do not pay for owning the software itself but rather for using it. They use it through an API accessible over the Web and often written using Web Services or REST .
The term SaaS has become the industry preferred term, generally replacing the earlier terms Application Service Provider (ASP) and On-Demand .
Image, video or audio file uploads of user-generated content
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Measuring Web 2.0 Actions Your Website Web 2.0 features you can add to your website that reflect a user’s behavior once they begin interacting with those features. Source: MarketingSherpa: Measuring Web 2.0: How to Promote the Features That Get the Best ROI