So, lets begin with the most fundamental of concepts. What is the difference between RGB and CMYK? This usually generates a blank stare...or superficial answer like "RGB is for your monitor and CMYK is for print." OK...but what is the difference? The answer to this question forms the groundwork for understanding all subsequent print color concepts.
Look at something red on your monitor. Now look at something red that is near you (not on a monitor or screen of any kind). What is the difference between them? Before you launch into explaining how one is darker, or more "brick colored" or whatever, I can tell you that you're already on the wrong track. What is the fundamental difference between those two sources of red? Quite simply, it is that the red on your monitor is projected light, while the other red is reflected light. In fact, the red from your monitor is red and the ink on the other object is everything but red. The ink absorbs all the colors that are hitting it and reflects only the light that you're perceiving as red. Any light source is comprised of three colors, red, green, and blue, and it is the intensity of these colors added together that give us color, thus visible light is an additive synthesis. Print, on the other hand, is subtractive, in that it is removing light that is hitting it and returning that which is not absorbed. So, why are things on your monitor always so much brighter than in print? Simple, because the colors in the printed piece are "left over" light that has not been absorbed by the ink. A muted version of visible light, so to speak.
This goes a long way to forming the groundwork for things like gamut and color separations which will be addressed in later posts.