CMYK is the oldest, color reproduction, print method. Using cyan, magenta, yellow and black ink, screen printers can print designs on white or light-colored garments. For users skilled in CMYK file preparation, Photoshop, and screen creation, this is still a viable print process for light-colored garments. CMYK is implemented less because simulated printing is more dominant.
Spot color is the most common print technique in the industry, and has sometimes been called the screen printer's best friend. Knowledge of creating, applying and handling spot colors is essential to any screen printer. Logos, text, cartoons, line art, and more are all styles spot colors excel at reproducing. Spot color designs are usually created through the use of vector graphics programs such as CorelDRAW and Adobe Illustrator. There are many programs that can create spot colors. Spot colors (such as Pantone or custom mixed) can be printed as solid shapes at 100% tint or halftones by changing their tint value away from 100%.
Simulated Process is a method used to reproduce almost any image on the market. It does so by overlapping and blending colors in a method similar to CMYK through using halftones and spot colors and/or PMS colors. Essentially, it's a way of taking an image that historically would have been printed via CMYK, but instead uses more opaque ink to create a more stable color and production friendly ink that can print almost any design for any garment. Designs that are too difficult to separate in a vector program, raster images, photographs, complex tonal illustrations — all doable with Simulated Process.
Check out this article if you'd like to learn more about these processes.