system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What are the main differences between existentialism and nihilism?
gpt
Nihilism wasn't really an actual school of philosophy, there may have been some contemporary nihilists who use the label for whatever reason, but historically it was more something you said about schools of thought you disagreed with if you felt that what they claimed as the grounds of truth and/or morality wasn't sufficient.   Nihilism can mean several different things, moral nihilism, nothing is either good of bad, epistemological nihilism, nothing can be known, or ontological nihilism, nothing is real or exists. 

Existentialism was a movement that developed around the first half of the 20th century, carrying a lot stuff over from some 19th century philosophers.   The name comes from the notion that "existence precedes essence", that is we are born into the world before we have a purpose, before we having meaning, and so we are free to find meaning in life.  Its not that there is no meaning, its just that people aren't tools, they're not made like a hammer with a purpose of pounding nails.  Existentialism has a notion of humans as radically free in the world, and ultimately responsible for it, the choice to keep living is a choice to in a way endorse the world.  Existentialism focuses on human's having choice, and authentically expressing themselves as opposed to acting in 'bad faith', bad faith meaning denying that we have a choice and that we are responsible because it allows us to conform more comfortable or massage our egos.