Web 2.0

Web 2.0 was originally coined by Tom O'Reilly and is a completely new level of internet websites above Web 1.0, which allows users to interact with the site they are on and other people involved in viewing. This has completely changed the way we think of websites as they are now not just a source of information but also a source of connectivity and give us huge power to alter them to our own desires.

Some examples are:
Wikipedia
Youtube
Facebook