I'm a photographer, who dabbles in graphic design from time to time as well. What are the differences between the various color spaces?
RGB is a additive, projected light color system. All colors begin with black "darkness", to which different color "lights" are added to produce visible colors. RGB "maxes" at white, which is the equivalent of having all "lights" on at full brightness (red, green, blue).
CMYK is a subtractive, reflected light color system. All colors start with white "paper", to which different color "inks" are added to absorb (subtract) light that is reflected. In theory, CMY are all you need to create black (applying all 3 colors at 100%). Alas, that usually results in a muddy, brownish black, so the addition of K (black) is added to the printing process. It also makes it easier to print black text (since you don't have to register 3 separate colors).
Most screens (computer, phone, media player, television, ect) are RGB (e-ink screens being an exception), the pixels have little subpixels that just show red, green or blue.
Most printers print in CMYK color (though some photo printers will print with expanded colors beyond those 4).
So if you're ever doing something for a screen, use RGB, if you doing something for print, use CMYK.
Update: Please keep in mind, that you can't display the exact same colors in RGB and CMYK.