Libertarianism sees the government as an aggressor against rights.
Liberalism sees government as the protector of rights, occasionally to aggress.
Most of the differences are derivatives from that.
I don't believe that the government protects my rights, especially due to it's incessant infringement upon them. I believe I should be allowed to declare myself and my land sovereign and independant from the country that thinks it owns my land. You could sum it up as Liberalism + belief that governments should not stop people from exercising their right to secede from any government and govern themselves.