In order to understand cross process developing, you first have to understand a bit about film in general.
Everything I describe in this section is with the caveat that it’s the general technique and material. There are always alternate ways of doing pretty much everything.
So, with that being said, there are two types of film. The film examples shown up above, even though they’re in different sizes (35mm and 120), are both print film. This film is developed with a C-41 process – a certain set of chemicals which let that film turn into a transparent image. Light is then shined through that image to touch reactive paper which makes the print.
The other type of film is slide film. With the slide film, an E-6 process is used. It’s an entirely different set of chemicals. These let the film itself turn into slides. There is no “second step” with slide film to make the slides. The film is chopped up and becomes the slides. So while traditional print photography leaves you with negatives so you can print out lots of prints, with slide photography your slides are the end product. You get one and only one slide per image.
So, with that being said, what if you start with slide film but then process it as if it were print film? I.e. you take photos with the slide film. But instead of processing it with E-6 chemicals and chop the film into squares / rectangles, you process that slide film with C-41 chemicals? So you can then make prints out of it?
The result is that the colors really pop. The contrast becomes quite strong. This creates beautiful results with high-color images.
If you haven’t tried creating cross processed images, I highly recommend it. Buy yourself some slide film, let your developer know that you want to process it as regular print film, and enjoy the results!