The primary claim of Liberalism is that governmental power is necessary, but that it should be distributed as widely as possible, in an interlocking series of checks and balances.
The primary claim of most of the people I have known who have called themselves Libertarians is that governmental power is NOT necessary, and that it should be as little as possible, and ideally something close to zero. This basic position could also be called Anarchism, where the idea is not a lack of order, but rather as many orders as there are individuals, and groups of individuals choosing to associate for a common purpose, for whatever length of time.
Philosophically, I have argued that Liberalism rests upon the idea that all ideas which do not cause harm to others are to be treated legally as equal. Morally, it stipulates no absolute principles other than the desirability of peace and order, as embodied in the principles of life, liberty, and property (and the pursuit of Eudaemonia, if we follow Jefferson).
Libertarians, on the contrary, DO posit an absolute principle, that governments have no claims on the freedom of individuals, and should not, in principle.
Practically, as an example, where a Liberal will see drug law as a matter for local custom, a Libertarian will view ALL laws restricting drug use as invalid morally. Where a Liberal would leave the matter up to the States, a Libertarian will seek to abolish all drug laws on a national basis.
These thoughts may be helpful for some.
I can’t resist adding that virtually every self-declared Libertarian I have ever met either smoked pot or read Ayn Rand with a devotion approaching religious adoration.