Props, gimmicks, gaffs.. what's the difference?
 

Someone actually asked me about this and ended up getting into a little argument with me as he thought it was too confusing. It's pretty simple though, so allow me to elaborate and share with all of you:

Props
Every single object you use.

Gimmicks
Extra or special props/materials your spectators should not be aware of (both clean and dirty ones).

Gaffs
Special/modified props (dirty ones).

This means that gimmicks aren't neccesarily gaffs, but gaffs are always gimmicks. For instance:
Props: coins and shell | Gimmick: extra coin and shell | Gaff: shell

I'd like to add that clean gimmicks can be ungimmicked. This means that there is a clean-up involved which means that the extra prop in question ends up where it originally belongs.

Hope this helps! ^_^