There are a lot of differences, the big ones are:

More acute seat tube angles. This allows you to save more energy for the run because it forces you to only use part of your leg for the bike.

More aerodynamic. This is so when you go down into your bars, you have the most aerodynamic position available to cruise in.

You'll often ride faster on a tri bike than a standard road bike over the long distances, but that's because of the aero position. Most that I have talked to say road bikes are more comfortable over the long haul though.

James