CMYK (Cyan, Magenta, Yellow, Black) color mode is used for print design. This includes logos, business cards, stationary, illustration, packaging and any other designs used for print.
RGB (Red, Green, Blue) color mode is for anything that is computer-based design. This includes websites, apps, banner ad and any other design created for electronic use.
For more information on CMYK vs. RGB, see our blog post, Correct file formats: RGB and CMYK.