A spark gap is an early form of an automated "switch" so to speak. The basic idea is that it is a break in a circuit. When the voltage (or potential difference) accross its terminals is large enough, the air between the terminals becomes ionized and the circuit is completed through electrical arcs.

Spark gaps have some very interesting properties and some interesting design problems. Ideally, the air acts as a switch, turning "on" when the voltage is high enough, and then off when it falls too low to sustain the plasma.

In practice this is not the case. The sparking heats up the air, and changes its content by making alot of ozone. Warmer air is more easily turned into a plasma, and this subsequent fireings happen at slightly lower voltages.

The shape of the electrodes also matters. Many new tesla coil builders have made their spark gaps from 2 nails that are mounted such that the tips face eachother. This design ignores the fact that electrons will spray from the point, into the air (a fact that doesn't mean much at lower voltages, but matters in such a system).

Spark gap transmitters have the property of cutting accross many frequencies at once. Thus they have traditionally been used on ships for emergency use. The idea being that every radio within range to pickup their distress call will hear it, no matter what frequency they were listening on.