You aren't being propelled around the track by a motor or pulled by a hitch. Once you're underway, different types of wheels help keep the ride smooth. Running wheels guide the coaster on the track. A final set of wheels keeps the coaster on the track even if it's inverted. Compressed air brakes stop the car as the ride ends.
Roller coasters can be wooden or steel, and can be looping or nonlooping.
You'll notice a big difference in the ride depending on the type of material used. In general, wooden coasters are nonlooping. They're also not as tall and not as fast, and they don't feature very steep hills or as long a track as steel ones do. Tubular steel coasters allow more looping, higher and steeper hills, greater drops and rolls, and faster speeds. Read more about their history.