Like New: Close to new but may have been played a small amount, but less than a guitar sold as "new" by a music store; may have case candy

Mint: Played but is in close to perfect condition with no blemishes or issues of any sort; similar to a guitar sold as "new" by a music store; may not have case candy 

Near Mint: Has been played several times, but has no obvious imperfections or issues; may have small pick swirls on pickups, etc.; similar to a guitar sold as "new other" or "demo" by a music store 

Excellent: Definitely has been played, but no significant noticeable wear (clean frets; may have minor pick wear, minor scratches, etc.)

Good: Definitely has been played, and shows generally minor but noticeable wear (from pick, scratches, on frets, buckle rash, etc.), with no effect on playability or sound

Fair: Noticeable and significant wear, including dings, scratches, finish wear, buckle rash, repaired cracks, and/or other (noted) problems, with no effect on playability or sound  

Poor: Guitar is in Fair or worse condition cosmetically, AND may also have serious problems with neck or electronics [NOTE: I don't buy or sell such guitars.] 

NOTE: Reasonable people can differ as to how Condition should be defined, and how guitars should be graded.  Elderly Instruments, a quite respectable retailer, grades an instrument differently.  I provide one overall Condition grade.  Elderly, however, may provide a grade something like "excellent condition, except repaired crack on headstock."  I would probably give that guitar a grade of either Good or Fair, depending on whether it had other noticeable flaws.