Because they are so often mentioned in the same breath, existentialism and absurdism can look like one philosophy wearing two names. They are not — though they share a birththplace. Both begin by refusing the comforting idea that the universe comes pre-loaded with meaning, purpose, or a plan for us. Strip that away, and a question opens: given that no meaning is handed to us, what should we do? It is on the answer to that question that the two traditions divide.
Existentialism's reply is constructive. If there is no given essence, then we make ourselves — Sartre's famous formula that existence precedes essence. We are radically free, and with that freedom comes the weight of authorship: we are responsible for the meaning we create, and there is no one else to blame for the self we become. Meaning, on this view, is genuinely available. It is just that we have to build it, through committed and authentic choice, rather than receive it.
Camus' departureThe honesty of the absurd.
Camus, who notably resisted being called an existentialist, thought the constructive move was subtly dishonest. The human situation, he argued, is defined by the absurd: the permanent collision between our craving for meaning and a universe that will never supply it. You cannot dissolve that collision by manufacturing meaning any more than by leaping to religious faith — both, for Camus, are evasions, ways of pretending the gap has closed when it has not. What is left, once you refuse both escapes, is revolt: to keep living fully, lucidly, and defiantly in the face of the absurd, without appeal.
That is why his emblem is Sisyphus, condemned to roll a boulder uphill forever, and why the closing line of his essay insists we must imagine Sisyphus happy. The existentialist would tell Sisyphus to find or make a meaning in the task. Camus tells him something stranger and harder: there is no meaning in it, and the task is still worth doing with your whole heart. Whether that is braver or merely bleaker is the choice the comparison above puts in front of you.