In the UK, a 'jobsworth' is a person who exercises their power by enforcing petty or pointless rules. It comes from the phrase "It's more than my job's worth...", meaning that they could be fired for breaking the rules, but the connotation of jobsworth is that actually, they're just being a jerk.
Jobsworth, Jobsworth, It's more than me job's worth,
I don't care, rain or snow,
whatever you want the answer's no,
I can keep you waiting for hours in the queue,
and if you don't like it you know what you can do.
-- Jeremy Taylor, Jobsworth, 1973
Jeremy Taylor is often credited with coining the word, although he did not use it in the modern sense, but rather as something that a Jobsworth would mumble. The current usage was popularized by the BBC television program That's Life!, which featured a "Jobsworth of the Week" segment, highlighting cases of unnecessary petty rule-enforcement. The phrase has lived on in the popular vocabulary since that time, although has not managed to cross over to America.
In modern usage, it may also mean any interfering person, including senses more akin to 'snitch'. However, the original meaning of 'petty official' remains the most common and best recognized usage.