Existentialists share the belief that philosophical thinking begins with the human subject—not merely the thinking subject, but the acting, feeling, living human individual. In existentialism, the individual’s starting point is characterized by what has been called “the existential attitude,” or a sense of disorientation and confusion in the face of an apparently meaningless or absurd world.
existentialism: a chiefly 20th century philosophical movement embracing diverse doctrines but centering on analysis of individual existence in an unfathomable universe and the plight of the individual who must assume ultimate responsibility for acts of free will without any certain knowledge of what is right or wrong or good or bad
-Based on Wikipedia’s entry and Webster’s, respectively.
Nihilism is a philosophical position that argues that existence is without objective meaning, purpose, or intrinsic value. Nihilists generally assert that objective morality does not exist, and that no action is logically preferable to any other in regard to the moral value of one action over another. Nihilists who argue that there is no objective morality may claim that existence has no intrinsic higher meaning or goal. They may also claim that there is no reasonable proof or argument for the existence of a higher ruler or creator, or posit that even if higher rulers or creators exist, humanity has no moral obligation to worship them.
Again, according to Wikipedia.
I suppose one of the main differences is that nihilists believe that there is no truth and since truth doesn’t exist, nothing does. Period. The end. As to where existentialists might not be as quick to point out what truth IS, or for that matter ISN’T. Instead they don’t rely as much on logic and deduction, truth and fallacy, but instead more on a persons inherent valuation of things. One’s intuitive responsiblity and will towards things.
Very similar in their initial perspectives, but they differ very much in how they progress from one rationale to the next.
Look, also, to the root words themselves: Existence and Nothing. Pretty much sums them up.