Existence precedes essence

Existentialism in three words.

Basically, it means that humans do not possess any intrinsic identity or value and how we choose to create value and meaning in our lives is up to us. As how Buddhist philosophers would put it, the self is empty in the sense that it lacks an intrinsic essence. Perhaps the difference between Existentialism and Buddhist philosophy is how they approach this issue.

Existentialism proceeds with focusing on authenticity which is descriptive in the sense that one could go about various ways to live authentically. While Buddhist philosophy (depending on which tradition) proceeds with different sets of guidelines such as the Eightfold Noble Path, Four Brahmaviharas, Six or Ten Perfections, etc. In this sense, the latter is more prescriptive than existentialism.

This is not to say that one approach is better than the other, as ultimately how we apply such principles depend on the situation and context.