what is the difference between 16 gb and 32 gb on ipads?

The latter has twice the mass storage of the former - it will allow you to store that much more stuff (apps, music or other data files).

Other then the amount of mass storage on them, the are identical devices. Think of it as buying the exact same computer, only with a larger hard drive.

16GB vs 32GB vs 64GB: Which new iPad storage capacity should you get?

http://www.imore.com/2012/03/08/16gb-32gb-64gb-ipad-capacity/

How much content will fit on my iPod or iPhone?

http://support.apple.com/kb/HT1867

How Much iPad Memory Do You Need?

http://ipad.about.com/od/iPad-Tutorials/a/How-Much-iPad-Memory-Do-You-Need.htm

Also, the 32GB costs more.

 Cheers, Tom 😉