Whilst I have a rudimentary understanding of these two photography options, I had a quick scan of Google.com for an explanation. Here are some answers:
Definition of RGB and CMYK
RGB: Uses the computers colors (Red, Green, Blue) and is the best option for the web or electronic publishing. Also called additive colors.When defining color spaces, use sRGB for web and Adobe RGB for printing, in most cases avoid the other RGB colorspaces unless you are doing specialized tasks that take advantage of these. Pro Photo is another useful colorspace but many of the colors can’t be reproduced on an inkjet printer.
CMYK: Color printing presses print with 4 ink colors Cyan, Magenta, yellow and black. If you look at a magazine close up you will see tiny patterns of dots, these dots are arranged in different patterns and sizes to fool the eye into seeing colors that are not really there, e.g. brown etc.
This method is used in most full color printing. I would only use this if you are going to a press. The disadvantages are, the image is a third larger than RGB, and you will lose some brightness as CMYK does not support as wide a tonal range as RGB. Also called subtractive colors.
Correcting Colors in CMYK and RGB
Although you can perform all color and tonal corrections in RGB mode and most adjustments in CMYK mode, you should choose a mode carefully. Avoid multiple conversions between modes, because color values are rounded and lost with each conversion.
~ RGB images to be used onscreen do not need to be converted to CMYK mode.
~ CMYK images that will be separated and printed do not need to have color corrections made in RGB mode.
If you must convert your image from one mode to another, perform most of your tonal and color corrections in RGB mode and use CMYK mode for fine-tuning. The advantages of working in RGB mode are:
* You can save memory and improve performance because there are fewer channels.
* RGB has a wider range of colors than CMYK, and more colors are likely to be preserved after adjustments.You can preview composite CMYK colors and separation plates using the CMYK working space in the Color Settings dialog box. Or, you can preview colors using a custom CMYK color profile.
You can edit an image in RGB mode in one window and view the same image in CMYK colors in another window. Choose Window > Arrange > New Window For (Filename) to open a second window. Select the Working CMYK option for Proof Setup, then choose the Proof Color command to turn on the CMYK preview in one of the windows.
Using consistent color settings and the right profiles, Photoshop can convert images from RGB to CMYK successfully. (The process can even be automated through batch processing and scripting options.)
Photoshop, however, is not the only option for desktop-level color conversion.
GIMP Plugin
BlackFiveServices.co.uk provide a plugin providing rudimentary CMYK support for The GIMP!
I’ve not tested this, so can neither confirm nor trust the validity of their claims about this plugin’s ability. Install at YOUR own risk.
That’s enough! There should be enough information in there to help even the newest to photograph manipulation!
Creativity & Artwork of Stephen Mitchell 
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