CMYK stands for Cyan, Magenta, Yellow, and Black, also known as process colors. These are the colors used by printers on most offset printers. It is important to make sure that all images (jpg, eps, tiff files etc.) are converted to CMYK. This is to ensure that all colors come out like you are expecting.
Most images taken from the web are RGB (Red, Green and Blue). If an image is taken from the web and used on a print project make sure the image is converted to CMYK before sending the files to a printer. Changing an image can be done with any photo editing program. Most used is Photoshop but there are also open source programs that are free and can do the same thing. You can find a good list here 10 Excellent Open Source and Free Alternatives to Photoshop.
If you are using spot colors just make sure these are converted to CMYK as well, unless using a specific spot color, the printer needs to be notified so they can match it. My recommendation is to just convert everything to CMYK, this is the safest and easiest way to make sure your printer has what they need and the finished product will look like what it should.