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.