The differences are many and varied. Too many to cover in a simple forum post. Read the release notes and release announcements for each version and google the different desktop managers (Gnome, KDE, LXDE, Xfce, Fluxbox) and code bases (Ubuntu and Debian) and you will begin to see the differences.
The best way to see the differences is to download each version and run the live desktops to compare them.
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