Excellent
An excellent trade-in looks, well, excellent. It should be in tip-top mechanical condition, the engine compartment should be clean and leak free, the paint should be glossy and the body and interior should be free of any wear or visible defects. The tires must be the proper size, and be in new or nearly new condition. If you aren't sure if your vehicle is in excellent shape, stop by your local dealership for an inspection. We're excellent at them.
Good
A car in good condition should be free of any major defects and have no major mechanical problems or rust. Tires should match and have substantial tread left and the vehicle should have a clean title history. You'd sell it to your own mother with a clear conscience. If you aren't sure if your vehicle is in good shape, stop by your local dealership for an inspection.
Fair
A car in fair condition has a few mechanical or cosmetic defects, but is still in safe running condition. The paint, body and/or interior need work from a professional in order to be sold. A clean title history is assumed, but even after significant reconditioning this vehicle may not qualify for the Blue Book suggested retail value. Vehicles in this category often have nicknames like "Old Faithful" or "Betty."
Poor
This means your trade-in has severe mechanical and/or cosmetic defects and may be in questionable running condition. It may have problems that can't be readily fixed, like major rust. You should probably have it independently appraised to determine its value. Basically, we're talking about the kind of car you wouldn't hesitate to park, unlocked, overnight in a not-so-good neighborhood.