Spirochetes are spiral ("spiro") shaped simple, extremely thin bacteria that move like a corkscrew (with the aid of a flagella). Two well known examples of spirochetes are Treponema pallidum, the cause of syphillis, and Borrelia burgdorferi, the cause of lyme disease. The image below is an example of a Spirochetes bacterium.
On the right is an unfortunate individual stricken by Lyme disease. Spirochetes do not seem to serve much of an ecological purpose besides preying off of host species like us.