Like anchors, lookaround groups match at a certain position rather than certain text. Lookahead will try to look forward at the current position in the string, while lookbehind will try to look backward. If the regex tokens inside the group can be matched at that position, positive lookaround will succeed, and negative lookaround will fail. If the regex tokens cannot be matched, positive lookaround fails and negative lookaround succeeds.
Lookaround is mainly used to check if something occurs before or after the match that you're interested in, without including that something in the regex match.
Since regular expressions normally cannot be applied backwards, most applications only allow you to use fixed-length regular expressions inside lookbehind. Some regex flavors don't allow any quantifiers, while others allow quantifiers as long as they're not "unlimited". The JGsoft and .NET regex flavors are the only ones that allow full regular expressions inside lookbehind.