You hit the nail right on the head Naughtylady. The cheaper units, and a lot of the expensive ones, will tell you what part is reading a problem but not what the problem actually is. The O2 sensor is the best example. It reads a rich condition in the exhaust and sets off the "Check Engine" light. But what's causing the rich condition? Bad injectors? Bad spark plug? Weak ignition coil? Plugged air filter? One of the best features to look for is one with "Instant Monitoring". That will measure the operating peramiters of most key systems. Many code readers offer a definition setting to translate the code into layman's language. A Haynes manual does the same thing.
Hey Ronnie. You're not a mechanic on the side are you?