What is 2G and 3G spectrum in Telecom?

I wont be able to explain it completely but will give two links that can help you to understand it

http://abcgeorgiesamuel.blogspot.in/2010/11/what-is-2g3g4g-spectrum.html

http://www.telcoma.in/en/wp-content/uploads/2010/09/3g-spectrum-india1.jpg

The first link is a blog and second is the allocation of 3g spectrum in India based on frequencies