Cross-processing (also known as ‘x-pro’) is the procedure of deliberately processing one type of film in a chemical solution intended for another type of film. As particular chemical solutions are optimized for specific kinds of film, you will get unpredictable and interesting results when they are combined differently.
Before anything else, let us tell you about the different chemicals used for the 2 most common types of consumer film.
Colour negative film uses C-41 chemicals for processing. You will get negatives as a result of this type of processing.
Colour slide film (also known as ‘reversal’ or ‘transparency’ film) uses E-6 chemicals for processing. You will get slides as a result of this type of processing. Think of slides as the slides that your grandparents used for slide projectors to bore everyone with their photos from their last vacation!
Cross-process Now comes the fun part. When you cross-process, you use the chemicals for colour negative film with your colour slide films, and vice-versa. So, when you cross-process colour negative film you process it using E-6 chemicals. And when you cross-process Slide film you process it using C-41 chemicals.
This info is from the Lomography webside crossprocessing.info