Free loop spaces of manifolds have very rich algebraic structures on homology. Their study, named string topology, has received a lot of attention since Chas and Sullivan's seminal work in 1999. To properly discuss these various algebraic structures, one needs the language of operads. In a first part, I will introduce these notions, without assuming any familiarity with homotopy theory.
Then in a second part, I will describe the brane action, a procedure due to Toën which constructs "universal" algebras on certain operads. I will end by mentioning my work generalizing this construction and its applications to group actions in (higher-dimensional) string topology.