The OBD2 standard allows for multiple electrical interfaces, which complicates the hardware used to interface with the vehicle. Equipment and Tool Institute has a nice interface by manufacturer page. Scantool.net also has a nice page listing vehicles and OBD2 protocols, and another describing a method of determining interface by connector pin layout.
Here is a brief summary of the most popular electrical interfaces:
ISO15765 (CAN) - Controller Area Network, a new higher speed interface, just now being implemented
ISO9141-2 used mostly by Chrysler and the European/Asian imports
ISO14230, or KWP2000 (Keyword Protocol) shares the electrical portion of ISO9141 standard, but defines it's own commands.
PWM J1850-41.6 (41.6Khz Pulse Width Modulated) used mostly by Ford
VPW J1850-10.4 (10.4Khz Variable Pulse Width) used mostly by GM vehicles
Here is a description o fthe SAE J1850 standardhttp://www.interfacebus.com/Automotive_SAE_J1850_Bus.html