Generative art is art that has been created through randomized or semi-random processes which run independently of the artist. These processes are typically computer based using procedural generation techniques, but could refer to real world processes which are produced mechanically. Work where the artist exaggerates randomness in their own techniques, such as paintings by Jackson Pollock are not usually seen as generative art.
Generative art usually refers to the visual arts, but not exclusively. Literary techniques such as "Cut-up" poetry are very similar to Markov chains and Mad libs are often used to create parody plot generators for genre fiction.
PCG Wiki References
Images on Flickr
The following images on Flickr have been generated using generative art techniques: