Dragon Ball GT is NOT the "third season" of Dragon Ball. Dragon Ball GT is the "third series" of the popular Dragon Ball Franchise. Originally intended to end with the (final) death of Son Goku after defeating cell, the franchise produced a few movies where Goku temporarily returned.
However, since the franchise was so profitable the producers decided to continue it with another series, called Dragon Ball GT. The letters "GT" do not really stand for anything, though some speculate that they are in reference to two of the main characters, Goku and Trunks.
After coming back to this dimension (ie coming back to life) Goku is accidentally tranformed back into a small child, as in the original Dragon Ball, complete with tail. He then goes forth, with Trunks, son of Vegeta and Bulma, as well has his granddaughter, Pan, to try to find the Galactic Dragon Balls, to return him to adulthood.
The show is much more adventurous, than Dragon Ball Z, and focusses less on Gargantuan Battles to Save the Universe. Similar in spirit to the original Dragon Ball, Dragon Ball GT is about the adventures of Goku and company, as they explore space, and get into zany adventures.