To snitch on a person means to tell someone in authority that the person has done something bad or wrong.
e.g. She felt like a fifth-grader who had snitched on a classmate.
A snitch is a person who snitches on other people.
If you snitch something, you steal it quickly and quietly.
e.g. Before I'd finished reading it, she snitched my copy of To Kill a Mockingbird.