that saddle guide that the last user posted pretty much sums it up, however.. if you were to buy a saddle, I would go with a barrel saddle.
I have a billy cook barrel saddle [its nothing special just a basic barrel saddle]
I have owned it for about 5 years now, I ride in it at least 2 hours every day, and when I go to shows its in use almost ALL day!
Its a great saddle, and a lot of my friends who have ridden in it say that its like a "seatbelt" it really keeps you in the saddle.
Its the best saddle I have ever owned. =]
You can tell a gelding, you can ask a mare, but you must discuss it with a stallion.