L-infinity spaces and derived loop spaces

We develop further the approach to derived differential geometry introduced in Costello's work on the Witten genus. In particular, we introduce several new examples of L-infinity spaces, discuss vector bundles and shifted symplectic structures on L-infinity spaces, and examine in some detail the example of derived loop spaces. This paper is background for a companion paper, in which we define a quantum field theory on a derived stack, building upon Costello's definition of an effective field theory.