Originally Posted by landrover4
"Fixie" is slang referring to any bike that has a fixed rear hub, that is to say, it does not have a freewheel so anytime the rear hub is turning the pedals are also turning. You can convert any bike (road, mountain, tricycle etc) into a fixie but a "track bike" is built specifically around the concept of the fixed rear hub and the stresses which come with this type of use. The chain, for example, is 1/8 inch instead of the thinner and weaker 3/32 inch of road bikes, and therefore the rear sprocket and front chainring are also thicker and stronger. The front chainring, which if not strong enough, could possibly suffer from metal fatigue and fail under severe stress such as emergency stopping when the chain is used to lock up the rear wheel. Or when sprinting.
Regarding the frame, yes as someone else mentioned the bottom bracket is higher off the ground than on road cycles and usually the crank arms are shorter, like 165mm instead of 172.5 or 175 on most road bikes. This is due of course to the fact that you can't stop pedaling and therefore when leaning into a turn you must be certain that your crank arms do not touch the ground as the crank arms continue to turn.
The wheelbase is also shorter which lends more stiffness to the frame and depending on what kind of forks you have you may experience "overlap" as someone else stated whereby the front wheel may touch the toes of the rider when turning the wheel, but this is certainly not always the case. The front forks, however, have circular tubing instead of the usual eliptical tubing on road bikes. Again, this is for lateral strength when sprinting away from a stop. For this reason, also, there are special track headsets which are stronger. Stems and bars are usually steel as this is stronger than aluminum. Generally speaking, track parts are stronger than road parts because they take such a pounding, and therefore they tend to command more money. Just check ebay and you'll see.
In short, a fixie may have a road frame, road forks, road cranks, a road headset, a road stem, road bars, a road chainring, a road chain, and a road sprocket but a track bike would not unless someone messed with it and put road equipement on it, or didn't have the good taste or the money to do so. Also, there are no holes for brakes in either the fork or the frame.