I came across this definition at Wikipedia just now. It's a good read!
"Name calling is a cognitive bias and a technique to promote propaganda. Propagandists use the name-calling technique to incite fears and arouse prejudices in their hearers in the intent that the bad names will cause hearers to construct a negative opinion about a group or set of beliefs or ideas that the propagandist would wish hearers to denounce.
The method is intended to provoke conclusions about a matter apart from impartial examinations of facts. Name-calling is thus a substitute for rational, fact-based arguments against an idea or belief, based upon its own merits, and becomes an argumentum ad hominem."