I always looked at snitching like this: if me and you do dirt, but I get caught, and I tell on you to get out of trouble and vice versa. That is snitching.
There's also willfully offering information pertaining to crime or criminal elements that you are also involved in, but you're giving law enforcement info on people in the same circles or connected to that circle. In exchange, they look the other way on your crimes.
Snitching, IMO, is basically being a Judas where you're willfully selling out friends and associates for a deal from law enforcement.
I never looked at people who witnessed a crime that they had nothing to do with cooperating with the law to catch a criminal.