The technology
Capacity
QR code standard will allow you to store a maximum of 4,296 alphanumeric characters. Bear in mind - the more characters you store, the more condensed the code becomes, which will make it hard to read by devices with poor optics.
Error correction
Every QR code has error correction mechanism built in. Depending on the level of error correction scanner can retrieve from 7% to even 30% of damaged code. The higher error correction level is, the less capacity is left for the actual contents of the code.
Licensing
Although QR code is a patented technology, the pattentee (Denso Wave) has chosen not to take advantage of it. Therefore, use of QR codes is free of any license, as they're defined as an ISO standard (ISO/IEC18004:2006).
Content
QR codes do not differentiate content, their readers do. Majority of the readers are able to decode alphanumeric text, URLs, phone numbers, email addresses and contact information (VCard). Additionally some of the readers will automatically recognize the content and react accordingly, such as opening URL in web browser app.
Readability
QR code is readable in independently of its orientation, since every code has fixed markers on its edges. The angle, however, does matter and it's best to scan QR codes placing the camera exactly in front of them, scanning from smaller angles might not work due to some of the bits being not visible.
Standards
There're many standard documents from around the world that include QR codes. There have been two models of QR codes from which only model 2 is used nowadays and is defined everywhere. Japanese comapny NTT Docomo has defined standards for encoding URLs, contact information and other data types.
Tips & tricks
Scanning distance
Keep in mind the smartphone optics will not perform as well as you expect them to. Therefore expect your QR codes to be scannable from around 0,5 meter. Placing them on billboards and such will make them unusable and users will get frustrated. Also, not everyone has an iPhone 4s with extraordinary camera, there's much more cheap feature phones that will scan QR codes too, but their cameras are much worse.
Label the code & incentivize
Do not just stick that QR code on the poster and expect people to scan it. You have to let users know what to expect when they scan the code. One thing you can do to improve scan rate is to incentivize people for scanning, there's a much bigger chance of anyone scanning the code if they get that 5% off whatever product you're advertising.
Apropriate content
Make sure the content to which the code is linking to is optimized for viewing on all kinds of mobile devices from one dollar feature phones all the way to high end smartphones. That means no flash and very light html content, bear in mind your users are on data network, they'll appreciate you not asking them to use a lot of their precious data plan.
Moving QR codes
Never, never and I really mean absolutely NEVER put QR codes on moving things. There's been a number of attempts to put them on for example a side of a bus... Users need time to take out their phone, launch the scanner app and finally to scan the code - this won't happen in 2 seconds, by the time person is ready to scan, the code can be already gone.
Good & bad places
Some places are better than others for placing QR codes. Bearing in mind it takes a while to scan, rather than putting a code in that busy passage way why not put in on the subway train where everybody is rather bored and is playing with their phones anyways.
Remember the primary use
QR codes are basically a copy/paste technology over-the-air, nothing more nothing less. We use it 'shorten' long URL's or text and rather than ask people to type it up using small keyboard on their mobile device, have them scan a simple code.
Not THAT ugly
Yes, most of them are black and white squares, but they don't have to be that way. Look at our code on the top of this page. Thanks to error correction algrithms you can change the colors of the code (keep in mind to make bits contrast from the background), make the edges rounded or even cut some bits out and paste your logo or a little symbol inside the code!
Be creative
Just because QR code can be a link, it doesn't have to link to a HTML website. How about link it to a coupon that actually incentivizes people for scanning the code? Why won't you get own mobile app and REALLY engage your customers, then the QR code can be a bridge, a shortcut to it.
Would you do it?
Always try to stand in a shoes of a regular person and ask yourself would I scan it? Does this convince me to take out my phone and spend 10 seconds of my time to scan that QR code? Is it attractive enough?
Tools & resources
QRHacker.com
With a little hacking around QR codes can get really pretty. QRHacker offers a variety of styling tools, starting with color manipulation, ending with pasting your logo into the code itself. Site also offers a store which could print freshly generated code on a number of promo items.
Likify.new
Likify is a nifty little tool that'll turn a QR code into a Facebook like button for your fanpage, so you can take fan aquisition onto the streets!
QRcode.Kaywa.com
Simple QR code generator, but hey - it works well! Select the content type from options like: URL, text, phone number or SMS and that's all.