Whats the difference between CMYK and RGB and why it can ruin your day?

Recently I had a client come to me with a common question and unfortunalty a common problem people run into when they get design work done by an amuetuer designer. Many times someoen looking for a business card design for example may jump online, do a quick search or maybe even post a quick add on craigslist looking for a designer to design them a business card and letterhead for there new business. The Designer does the job and then sends them over the file. Next you send your file to whichever printing place you are using to print your business cards and you sit back and wait a week for your cards to show up in the mail. The Day arrives, you open your box of brand new cards and the colors are completely off. Its not what the designer sent you so it must be the printing places fault right? Wrong. What happen is the designer showed you a file that was done in RGB color mode. This mode is made for computers to show the correct colors on your screen. But when you use an RGB file to print the colors come out different. Vise-versa, if you use a CMYK image online the colors will be off. RGB is for web and CMYK is for printing.