"Web 2.0" is an umbrella term for internet businesses that allow users to put content on the Web ("User Generaged Content" or UGC) is another term you'll hear. YouTube and Facebook are popular examples. Web 2.0 businesses include Social Networking sites, …
The name comes from the hope that this is the "second coming" of the internet. The first generation of internet businesses focused on selling to customers. Web 2.0 companies use use information from customers or provide services to users.
See O'Reilly: "What is Web 2.0?"
Corporations use blogs e.g.