In a nutshell, Web 2.0 is a term that captures the changes in the Web experience from viewing pages to participating in interactive, social, person-to-person applications.  It is also referred to as the “read-write web,” and some of the applications are referred to as “social software.”  While there is no consensus on a strict definition, you can get detailed descriptions from these sites.

Tim O’Reilly

http://www.oreillynet.com/pub/a/oreilly/tim/news/2005/09/30/what-is-web-20.html  

Wikipedia

http://en.wikipedia.org/wiki/Web_2.0

Richard McManus

http://blogs.zdnet.com/web2explorer/

These are some of the characteristics that McManus gives:

The underlying philosophy of relinquishing control

Giving up control and setting the data free

Web 2.0 is social, it's open (or at least it should be), it's letting go of control over your data, it's mixing the global with the local.

Web 2.0 is about new interfaces - new ways of searching and accessing Web content.

Web 2.0 is a platform - and not just for developers to create web applications like Gmail and Flickr.  The Web is a platform to build on for educators, media, politics, community, for virtually everyone in fact!

Here are some of the changes that Tim O’Reilly notes.   I've added comments in [] to clarify some of these distinctions:

• Ofoto [a service focused on uploading and printing digital pictures]--> Flickr [an online photo service that allows easy printing, sharing, collecting others' work, commenting and tagging others' work, quick publishing to blogs]
• Britannica Online --> Wikipedia
• personal websites --> blogging
• publishing --> participation
• content management systems [password protected online systems with a distinction between content creator and consumer] --> wikis
• directories (taxonomy) --> tagging ("folksonomy")

 I would also add:

• Internet radio [more technically complicated to create and synchronous] --> podcasting [infinitively easier, greater indexing features with rss and Itunes, and asynchronous and portable--in total, more convenient for creators and consumers.]
• ftp --> server-based authoring applications.

The chief technologies that relate to this movement are podcasting, rss feeds, blogging, wikis and tagging.  

To get hooked on Web 2.0 applications, try…

Bloglines (www.bloglines.com)

Flickr (www.flickr.com)

Seedwiki (www.seedwiki.com)

pbwiki (www.pbwiki.com)

Blogger (www.blogger.com)

Digg (www.digg.com)

Podcasting features of Itunes 5.0 or higher (http://www.apple.com/itunes/)

Image: Untitled by Babsi Jones