There's more than one, but the usual type is self generating (winding around a magnet) and plugs into the transmission tailshaft and looks at a multi-tooth gear on the tailshaft. Some have 20 or so teeth and some 40. There are only 2 wires and either can be connected to signal or ground.

I have one in an old TH400 I modified for electric speedo and the VSS in my '97 truck was just replaced recently.

Hope this helps some. Kitz