This is a very basic tip which can affect the quality of the printed publication. The difference between CMYK and RGB is as follows.
CMYK is made up of Cyan, Magenta, Yellow and Black. These are the colours that are used in the printing process. For example if you use a colour printer you will notice it usually has four cartridges one for each of these colours.
RGB is made up of Red, Green and Blue. These are the colours used by screen displays such as your monitor.
Basically all images that are to be used to for printed media should be in CMYK format and all images to be used for web format should be in RGB. You will notice that there are limitations with the range of colours that can be produced using CMYK, RGB colours often appear much brighter on the screen but these colours cannot be produced to the same brightness when printed.