Need help buying a road bike?

but i have a few questions:

-what's the difference between a road bike, touring bike and a triathlon bike? they look the same

-i'm 5' 11" so what would be the right size bike for me

-i'm looking to buy used so i can get more bang for the buck...so what should i look for as far as potential problems or things that wear out.

-and what would be a good used bike to buy for around $500-600

thanks

MV was right about the different classifications of bikes as far as road and touring go. With a triathalon bike, you can expect to have a different set up on handlebars, gear ratio, and geometry of the frame. Triathalon handlebars are also known as "aero bars" and the generally do not have the drop section which is common to standard road, touring, and cyclocross bikes. The geometry is going to be a bit more agressive (more leaned over).

As far as size goes, the size of the frame is greatly dependent upon the length of your leg (more than other measurements). If I had to guess, I would say that you are looking for a 57-59cm frame (19-20 inch). Just make sure that you have the proper leg extension when you are buying the bike (without the seat being a mile out of the frame).

As far as used bikes go, look for cromolly steel. There are a ton of old steel road bikes out there (especially with this new fixed gear craze) but you want to make sure that the steel is cromolly. AVOID High Tensile (or HIgh-Ten). Also, when buying an old steel frame, look for rust because this will be your biggest problem.

If you are looking to spend $500-$600, go with a new road bike. Many major manufacturers today offer entry level road bikes right around this range (I work in a bike shop). You can get a basic aluminum road bike which will exceed the quality of most (most being the key word) of the used bikes you will find out there.

Road bikes are made for speed; light and fast, geometry designed for aerodynamics. Two rings up front, 53/39 tooth combos are common, although they come in many variations.

Touring bikes are made for, well, touring. Loading it up with luggage and taking off for a few days/weeks/months. More upright geometry for all day comfort. Beefier frame, wheels, tires. A lot of them come with a triple crankset similar to a moutain bike, so you can gear down to carry all that weight up steep hills.

I am not exactly sure what the diff is with a tri bike, but I'm sure someone here will soon clue us in.

Cyclocross bikes are also in the mix. They have more room between the frame/wheels than a road bike for knobbie tires, but lighter frames than a touring bike. Generally a double crankset. Cross races involved riding on any surface; lots of dirt paths and obstacles designed to force the rider to dismount and carry the bike over/around obstacles. These bikes are sturdy by design(and fun as heck to ride!)