Using QR codes

A "QR" code is like a two-dimensional barcode.  It's a machine-readable diagram that represents some information.  In this case, the "machine" is a smartphone with a camera.  There are apps for iPhone and Android that will scan the code and take some action, which is usually to go to some website.  For example, the code here is a URL for a website that I have set up advertising a dog for adoption.  You (or your students) can use a smartphone to scan this QR code and open the encoded website.

The Android app that I use is called "QR Droid".  There are many equivalents for iPhone.  But to create the codes in the first place, I use this website: easyqrcodegenerator.com/free-qr-code-generator.
I've used this with lab instructions for students, with the idea that they could scan the QR code with their phone and then watch a video showing the lab procedure.  You could also use this to easily link to YouTube videos, Khan Academy videos, or anything else that you want the students to see.  Many textbooks at the college level now include QR codes within the text, that link the students to online content.  It's a good way to connect printed paper to online resources.