QR Codes (Quick Response Code) are like a UPC Codes but can store more information. They're used mainly on printed material to let people get to a website on their phone quickly without having to type an address.
The great thing about QR codes is that you can add tracking to them so you can see in Google Analytics how many people used the particular code. If you had an advertisement in three different magazines, you could tell which magazine sent you the most visitors, something you could never do with a regular print ad that listed your website's address on it.
Here's the steps to make one:
Generate the Tracking URL
First think about the page you want send people to. It may be your homepage, but ideally it's a page directly related to the material you're printing the code on, or a landing page created specifically for the advertisement. Copy that address, then use the Google Analytics URL Builder to create the extra long URL that contains the tracking codes.
Make the Long Google URL Shorter
QR Codes work better when the URL is short (which isn't the case with the extra long URL you just made) so take that URL from the previous step and use a URL shortener, like https://bitly.com or http://goo.gl
Finally Make the QR Code
So now that you have a nice short URL that contains all the tracking information, you can make your QR Code. There are many generators but the one I like is BeQRious.
Test, Test, Test
A good size to start out with on something people would hold in their hands is 1" wide with space around it. Once you get it on your design, print it out and test it on different kinds of phones and QR Code readers you may have. Make sure it's big enough that your phone can read it, and that it goes to the right page.