What is the meaning of the expression "that's apples and oranges."?

Question:

What is the meaning of the expression "that's apples and oranges."?

A male colleague of mine at work is always using the expression "that's apples and oranges," or "you're comparing apples and oranges." This guy is an engineer and has a reputation as being a logical thinker. I don't have the nerve to ask him what this expression means. Can someone please explain it to me.

Best Answer - Chosen by Asker:

It means that the two, or more, items that are being considered are not equal in some way. Although they are of the same type or variety, they are not the same in scope or features. Using the car as an example, when you say "why are these two same models and makes so different in price?" Well if one is a base model and the other is a fully loaded model, then you could use the term apples to oranges.
In science and technology, the differences can be very subtle and not too noticeable. Ask him what some of the issues are when he uses that phrase. "so whats so different?". Some times the differences are not an issue and you can argue the case anyway. Don't let anyone get away with that phrase too often. Some times people use it as a way to avoid a discussion in their favor. Especially if they don't know and they know that you don't either.