Well, photo-realism means just what the name describes, ie: an artist reproduces a scene or picture to exacting detail. Perfect reproduction.
Illustration has broader possibilities, with the artist implying the subject with his/her own flair. Here are a couple of examples in the tattoo world:
https://www.instagram.com/thedrowntown/
https://www.instagram.com/teresasharpeart/
https://www.instagram.com/tpenningtontat2/