There are a few differences between a road bike and a tri bike or TT bike.

1. Frame Geometry: A TRI bike has a much larger or steeper seat tube angle than a road bike which allows for a much more aerodynamic position for the rider.
2. TRI bikes tend to come with aero-bars already installed and the handlebars are designed to be used in the aero position.  Riders only sparingly leave that position.
3. TRI bikes are steady and are designed to best handle straighter, flatter terrain whereas a typical road bike will tend to be more nimble and responsive.
4. TRI bikes typically tend to be a little heavier than a good road bike.