What is a QR code?
The short answer is that a QR code is like a barcode that can be scanned by a smartphone and contains information, usually a web link. For a more detailed answer to this, see whatisaqrcode.co.uk
Why would I want to use a QR code?
Any time you want to be able to easily share a piece of electronic information with a smartphone user really. What I tend to use these for is sharing links (URLs) to video clips on YouTube or providing hints or additional background information for support or for extension to tasks and QR codes are an ideal way to do this. Not every student will have access to a suitable device but at WGSB the ratio appears to be somewhere between 1 in 5 to 1 in 2 students (from Year 7 -13) from my observations so small groups sharing a device is likely to work without a problem.
In my classrooms I tend to have a projector (although for the most part visibility is far from optimal) and most of the time, working speakers and I may want to show a short video clip for a starter or a longer clip that students will answer questions on in a longer activity. I show the clip once or maybe twice to the whole class but some students will not have taken everything in and will need or want to watch the clip again. Why not let them do so on their own device so they can watch just the bits they need to, as many times as they need to?
How do I go about using a QR code?
Let's look at an example: I recently put together an activity for a Year 7 lesson on the greenhouse effect and global warming. I found an excellent YouTube clip that I wanted to set questions based on.
I posted the link on Moodle so I, my colleagues and our students can all easily find this both in the lesson and later on if needed, the short link (found using the 'Share' option on YouTube) is http://youtu.be/HK8LLWSIIm4. This is worth including in your resource so students without the ability to scan a QR code have the option of typing in a short URL instead. To generate the QR code I visit a free generation service such as QRStuff.com and paste the URL for my YouTube clip into the relevant box, click 'generate' if required (not on qrstuff) then preferably try scanning the code onscreen to test it. Here's my QR code:

Download the image (give it a sensible name!) and you can then Insert the image into your worksheet or handout in the same way you'd insert any other image. The images can be pretty small but should have some clear space around them. Older and cheaper phones won't have brilliant cameras so could struggle if you make the image too small. If you have a long URL the QR code will be more complicated and will be harder to scan so consider using a URL shortener such as Google's offering or the 'qrs.ly' service offered by QRStuff (both are free). I've shared my example worksheet through Dropbox and you can get the file here to see how it looks
I posted the link on Moodle so I, my colleagues and our students can all easily find this both in the lesson and later on if needed, the short link (found using the 'Share' option on YouTube) is http://youtu.be/HK8LLWSIIm4. This is worth including in your resource so students without the ability to scan a QR code have the option of typing in a short URL instead. To generate the QR code I visit a free generation service such as QRStuff.com and paste the URL for my YouTube clip into the relevant box, click 'generate' if required (not on qrstuff) then preferably try scanning the code onscreen to test it. Here's my QR code:

Download the image (give it a sensible name!) and you can then Insert the image into your worksheet or handout in the same way you'd insert any other image. The images can be pretty small but should have some clear space around them. Older and cheaper phones won't have brilliant cameras so could struggle if you make the image too small. If you have a long URL the QR code will be more complicated and will be harder to scan so consider using a URL shortener such as Google's offering or the 'qrs.ly' service offered by QRStuff (both are free). I've shared my example worksheet through Dropbox and you can get the file here to see how it looks