Cross-Processing is traditionally associated with the technique of developing positive or negative film in the chemistry meant for the other type. This usually meant processing C41 (negative film)in E6 (slide film) Chemicals & E6 in C41.
The result is altered, often unnaturally saturated colors, and high contrast. It has been popular in fashion photography for years and has maybe been overdone on occasion... :-)
However, it's still a neat effect but for those who shoot digital this process can only be achieved through computer trickery. This group is dedicated to exploring, discussing and showcasing digital cross processing (xpro).