XP vs. Vista vs. 7- what are the differences?

This debate came up in a computer programming seminar that I attended. Huge debates on core differences. Really- what are these core differences? The UI design may be different, but what are the differences anyway between these versions of Windows?

Here is a comparison of the different versions
Compare Windows 7 Windows Vista Windows XP Features Offers Upgrades

Here is a comparison of the different editions
Windows 7 Pricing Buy Install Choose Edition Features Upgrades Offers

Check out this google search too for some good articles
windows 7 under the hood - Google Search

xp = old, but a trusty, reliable OS.

vista = no.

win7 = YES!

win7 = YES on 64bit with 8GB of RAM!