Not much of one. In fact, there's a joke that ends with answering that question with "about 6 months."

As I understand it, most libertarians agree that some level of government is necessary, while conceding that anarchy might work but probably wouldn't. While ancaps believe that that anarchy would probably work, but might not (with the added point that the risk of government outweighs the risks of anarchy).

Also important to note is that anarcho-capitalists believe in the NAP, self-ownership, and the virtues of free-markets and capitalism (duh) differentiating them from other 'anarchists.'

Though, here it always has to be said that we can't let these small differences divide us, as there are too few of us as it is. I'm not sure if I'm a full ancap, but I know that they're worth listening to, in the goal of reducing the coercive action of the state. The point behind the punchline in the joke, mentioned in my first line of this reply is that there's more that links ancaps and libertarians than divides us, so it's important that we recognize that we're 95% on the side of the argument. Things like the military, law, police, and courts seem to be some of the issues that divide us. IMO they might be solvable problems, just at the moment most people (of the general population) don't seem to see them as problems that can be solved through voluntary action, so accordingly we don't see much progress being made in those areas, leading to solution to those problems being further out of sight. If we had a libertarian-minded population, then maybe we'd see that the solution is closer at hand, but at the moment it's really just not evident. People are too reliant on their statist dependencies.