I was trying to think of a single sentence to sum up Libertarianism in an concrete way and came up with this:
Libertarians are folks who want the government to stay out of your bedroom, medicine cabinet and wallet.
I like it because it's true and:
1) you don't need to know any political terminology to understand it
2) it's a position that everyone can agree with (who wants the government invading any of these places when it comes to themselves?)
3) puts anyone opposed to Libertarians in a rather awkward position