Cross processing is a technique that was originally developed for processing film images. By processing the film in the wrong type of chemicals (e.g. processing film in slide chemicals), you could create a highly stylised image with drastically different colours and contrast.
However, cross processing film is a largely hit-and-miss affair which is very hard to judge, and requires a lot of experimentation to get the desired effect.
Nowadays we can simulate cross processing digitally in programs such as Photoshop, or any other which includes a Curves function. Digital cross processing has the added benefit that we can easily tweak the results until we get the effect we desire.[link]