2nd Generation vs. 3rd Generation

I am seeing numerous posts discussing 1st, 2nd, and 3rd generation iPods, but nothing that actually explains what the difference is between a 2nd G and 3rd G.

I have a 32GB iPod touch in which I purchased in July of 2009, and according to Apple, my iPod touch is 2nd generation.

So, can someone explain what is different between my iPod touch and one purchased in September 2009 that is considered 3rd generation?

Thanks in advanced.