Tetanus is an infection caused by Clostridium tetani. The name tetani/tetanus comes from the shape of this bacteria. It is rod shaped, but with a round spore at one end. This shape looks like a tennis racket, hence the name tetani. C. tetani lives prodominately in the soil. Humans contract the bacteria from the soil when something that is soil covered penetrates the body (such as a nail that was once outside). Contrary to common belief, the rustiness of the nail is not what we should be afraid of, but the soil covering the end of the nail!
When C. tetani enters the body, it starts making a toxin. This toxin enters the nerves of the central nervous system and prevents the nerves from being turned off (it blocks the release of inhibitory neurotransmitters, for those super nerds who are curious :P). When the nerves can't "turn off" the continue to fire and those in muscles can become paralyzed in an active form (i.e. the contracted form of the muscle).
One of the first muscles to be affected by the tetanus toxin is the masseter muscle in the face. This muscle allows us to chew our food and clench our job. As such, if the masseter becomes stuck in it's contracted form, a person will suffer from "lock-jaw".