It's all about the colorS we can see!

http://dba.med.sc.edu/price/irf/Adobe_tg/models/rgbcmy.html
Good information and clear illustrated explanation about human percievable color space(CIE model) as well as the relation among sRGB and CMYK Gamuts.
If you you are really into color space world, dig into CIE families in the links on the same page. Very informative stuffs.

http://www.techmind.org/colour/
This link offers both technical and easy to understand information guiding you into the world of colors. Great info!

http://en.wikipedia.org/wiki/CIE_1931_color_space
And of course, never forget this one from wikipidedia. :)