Web 2.0 is a buzz phrase for user interactivity on the Internet, this is a marketing term as much as a technical design or programming term and is short hand for allowing feedback and other forms of interactivity on a website.

You may be familiar with many forms of web 2.0 with out realising it, if you have used Ebay or Amazon then the feedback or review pages are basically web 2.0 content. 

New applications and technology that allow users with little technical knowledge to create their own webpages or submit content to existing websites, good examples of this include blogs, you can create a blog and update it as often as you like with little or no technical knowledge, you do not need to worry about such things as hosting or FTP, if you can use a word processing package, such as word then you can have your own blog.

Social Networking, Social Bookmarking, Customer Reviews, Blogs, Wikis, RSS feeds are all common uses for web 2.0 and all allow interactivity between the user of the website and the website itself, this can be great for content generation and sharing ideas or for leaving feedback and conducting customer research.