In this context, it's an intelligence/law enforcement term for "potential terrorist" or "terrorist supporter". An extremist group may have al-Qa'ida like philosophy or links, but hasn't actually don't anything yet. As an example, the 19 hijackers were extremists, right up until they siezed the planes. Then they became terrorists.
Also, "Extremist" is almost always shorthand for "Sunni extremist", unless otherwise specified. The term would not be used to describe a neo-Nazi group, for instance. It's become a very specific euphamism for terrorism.
That's a reasonable definition for law enforcement purposes- but it is startling to a civie...thanks for the explanation:)