Tarck is a track bike ridden on the street, usually with no brakes. It has to have track geometry and track drop bars. It usually has very agressive geometry. Tarck is quite different than a "fixie" in that a "fixie" usually is ridden with a brake or has riser bars.
See that fucker flying down Main St on his tarck bike? He's passing all of the roadies.
Tarck is the culture behind taking fixed gear bicycles designed for riding in the velodrome and riding them on the street. This subculture has a strong focus on color-coordination and fixed-gear specific tricks such as skidding or doing a whirly-bird.
"Did you see that kid doing barspinzzz on that tarck bike?"