Slang is not inherently offensive. The term usually denotes non-standard linguistic innovations restricted to a particular speech community and used to mark one’s membership in that community.
Typically slang is the speech of young people—words and phrases which mark the user as ‘cool’ or ‘hip’, familiar with the currently most fashionable artistic and social trends. But every in-group has its own slang. In the grain-processing industry, for example, warehouse receipts and shipping certificates are both referred to colloquially as ‘tickets’.
Slang should be distinguished from jargon, which is the formal usages or terms of art peculiar to a particular professional group.
Noob or n00b (formed after boob, in the sense ‘stupid and unsophisticated person’) was coined in the direct-dial bulletin-board communities which preceded internet forums. As SF. says, it is deliberately derogatory, employed to distinguish novice participants from the established senior users, masters of the medium. But it was balanced by a corresponding non-derogatory slang term, l337 (= leet, ‘elite’), for the established users.