Someone who tattles or informs another person's misdeed to authority figure. Usually about a minor offense.
Man: taking pictures in a store
Snitch: What are you doing?
Man: Taking some pictures of these items I was thinking about buying.
Snitch: runs to the store's management
Some guy is taking pictures of items in your store.
Store Manager walks up to customer
: Excuse me, sir, we don't allow pictures in our store. You'll need to leave.
Man sees the snitch as he exists the store. Looks at the snitch and says: Snitch!