His novels were concerned with deeply philosophic problems and curiosities. He is often referred to as the "godfather of existentialism", there is a lot of truth in this as he and Sartre were he pioneers of the revised investigation into phenomolical ontology (existence as it appears to humans).
He never referred to his own work as existential which although naturally a tad absurd helps to elucidate his isolation from the existentialist crowd such as Sartre. He never seemed committed or asssured of one theory but instead was interested in experimenting and disseccting the world philosophically as he saw it.
His huge influence also spread to the theatre through his concept of the "absurd" as outlined in his masterwork, "the myth of sysiphus" which starts with the preposition that as humans we live in a cold, non-responsive mateial world devoid of meaning, and since he was an atheist no leap of faith or god could provide consolation. From this thought he then considered suicide as a possible solution but instead wished to design a philosophy for existing in such an absurd and cruel world and therefore giving it meaning. His struggle with suicide, or arguable nihilistic thoughts, emanated throughout this work as the notion of solipsism (believing you are the only conscious being in existence, or some variety of belief that self-centered) did in Sartre's. And through this remarkable work the ideal of existentialism was constructed: to establish meaning in a meaningless world entirely through one's own mind and accepting and bearing all the pretty crushing responsibility this entailed.
This notion of the "absurd" was engendered into to the theatrical scene through the emergence of the "theatre of the absurd" pioneered by playrights such as Samuel Beckett and his dazzling "Waiting for Godot". This theatre despite adopting such heavyweight philosophical themes was always very amusing, often in an "absurd" way.
Following the "myth" Camus' next collection of essays was "the rebel" which set to describe and provide an account for what Camus identified as man's intrinsic existential virtue, which was rebellion against a corupt, evil or in same way philosophically undesireable present state in order to construct a better one. This collection took Camus 6 years to write and is an impressively thorough and insightful work. Although it is was not as revolutionary as "The Myth" it is a very worthwhile, and i found, very informative and educational piece. It gives an account of what Camus thought were acts of existential insurrection from Marquis de Sade's sexcapades through to the communist revolutions. As well as such historic descriptions "The Rebel" also describes acts of existential rebellion in art, literature etc. Camus also describes in a politically scientific way the progress of all such rebellions and guidelines by which suhc rebellions do not fall from grace into to tyranny as many did, (for example the emrerance of the Stalinist dictatorship soiling he aims of the intial rebellions, incidentally it was this critique of communism that cultivated the insurmountable rift between him and Sartre.)
Camus was no doubt one of the most prodigious and revolutionary thinkers of the 20th century and one who was partly responsible for the evolution of the philosophical period of Existentialism.
To those who are unfamiliar with Camus i recommend first reading "The outsider" which is a brilliantly accessible novel encapsulating his existential thoughts, then i recommed reading "The Myth" and finally "The Rebel". And after this read anything else that takes your fancy.