TheArtofRetouching.com - This Tutorial focuses on the basic differences between sRGB, Adobe RGB, ProPhoto RGB and CMYK color space. This video is only 12 minutes, and gives a quick overview on how each color space is designed to be used. It also gives a demonstration of how the image color can shift on different priced monitors.
sRGB -- Most cameras take the picture in sRGB. Monitors display the internet with sRGB. Visually, it has brighter colors than CMYK and Adobe RGB. However, sRGB has a tendency to display over saturated colors, which makes it terrible to try and print from. You will always be disappointed.
Adobe RGB (1998) -- While Adobe RGB does have a large range of colors than sRGB, it tends to display those colors a bit more muted. But, you know, it's all about how you handle the tool, before it can perform it's best for you. Keep in mind that a professional photographer that wants to print their images will likely have an easier time going to the inkjet printer, or convert down to CMYK, if they have been working in Adobe RGB. Mainly, because nothing can reproduce the vibrant colors coming from sRGB or ProPhoto RGB anyway. For more detailed explanation, please review this article on the difference between sRGB and Adobe RGB.
CMYK -- This is the industry standard to 4-color printing in magazines, newspapers, and other types of flyers. It also has the most limited amount of colors it can reproduce, it tends to have much duller colors and can sometimes display banding or morie's within gradients. You should only use CMYK when pring to print on a press. However, it is much easier to make advanced color corrections using CMYK tools, within RGB Color Space, like Selective Color. The reason is that RGB is a subtractive color space, and CMYK is additive color space.
ProPhoto RGB -- This color space was developed by Kodak, with an extremely large gamut designed for use with photographs. ProPhoto RGB should be used with 16-bit depth due to the posturization and banding issues that it can create if used with 8-bit depth. While this one has some of the widest ranges of color available, I caution it's use outside of a controlled environment of people who expect to receive a file with this profile attached. Follow the link to learn more detail about ProPhoto RGB.
This is a video Color Space -- sRGB, Adobe RGB, ProPhoto RGB and CMYK Tutorial. If you would like to learn more about Photoshop Tools and Palettes from a Professional Retoucher, I offer a 2 hour video explaining what each tool is, and how to use it's palette. If you would just like to watch other online videos, The Art of Retouching Studio offers many Photoshop CS5.5 Tutorials for Beginners and Advanced users.