"jerk" as a noun can be used in several different situations. It is typically only meant in a very insulting way when used by children. When used by adults it is often used in a teasing or flirtatious way, because it is not vulgar or a swear word. For example, if I start teasing my girlfriend about something, she might hit my lightly with the back of hand and tell me: "Stop being such a jerk!" When adults really want to insult someone, they will normally say something more vulgar like "asshole" or "dumbass."
Also, I agree with Dan Smith, do not use the term "jerk off", either as a noun or verb, unless you really know what you're doing. It does mean "wanker" in British or "Wichser" in German.