The Princess and the Frog is traditional 2-D hand drawn animation. It follows the story of a girl in New Orleans and will be based off of the fairy tale of the Prince who was turned into a frog I believe.
Rapunzel I believe is 3-D computer animation, and it will of course follow the fairy tale of Rapunzel.
Rapunzel is going to be the most beautiful thing that anyone has ever seen be produced by a computer.
The software (like most companies) has been developed in house to specifications set by Glen Keane and though it took Disney millions to get it to work right....it'll be the best investment they've ever made. This movie looks absolutley amazing.