Nihilism states that no matter what you do, it's meaningless. But how do you decide then, what to do?
A few years back I read "The Myth of Sisyphus" by Camus, in which he tackles the question, if suicide is the logical outcome of nihilism (which he finally answers by "no"). He calls his philosophy absurdism, but from what I know, you can consider it as a form of existentialism.
So is existentialism basically "pratical" nihilism? Or are there logical outcomes for one by nihilism itself?
Nihilism states that no matter what you do, it's meaningless.
Actually, it's a great deal more complex than that. There are a large number of conceptions of nihilism (a few of which are listed here), and they all share one important attribute-- it is almost always a term attributed to someone else, to argue against. You'll be hard pressed to find someone declaiming and defending nihilism per se.
So is existentialism basically "pratical" nihilism?
Well, existentialism certainly contains aspects often associated with nihilism, but I think it would be a mistake to reduce it to that-- most existentialists argue that since (for man) existence precedes essence, there is no essential, predetermined meaning to one's life, but rather that one must create meaning for oneself; this is not the same as arguing that "no matter what you do, it's meaningless."
So is existentialism basically "pratical" nihilism?
No, they are not the same.
Existentialism is not as narrow as it sounds, but always seems to have two major tenets --the first mentioned in the other answer (but not elaborated)-- which is that the argument that consciousness is the ultimate undeniable proof of existence (Descartes), is rejected, (or nick-named "existence preceeds essence"). Existentialists always find themselves already in the world, thrust into being, and consciousness is not the (a prior/a posteriori) priority.
Existenialism's other common trait (derived from the first) is the idea that one is ultimately responsible for one's own actions, because one is free to choose. This has been nick-named "to do is to be," if that makes it any easier to remember or understand. Nihilism rejects this entirely.
The confusion is probably because the only practical form of nihilism is likely "existential nihilism." Nihilism is the negation of meaning. That is not a fundamental characteristic of Existentialism. Consider that even Roman Catholicism is existential in nature, but hardly nihilist.
Also, it should be noted that Camus didn't consider himself an existentialist, like Heidegger or Sartre, but referred to himself as a moralist.
