What is a QR code?

QR code (abbreviated from Quick Response Code) is the trademark for a type of matrix barcode (or two-dimensional bar code) first designed for the automotive industry in Japan. Bar codes are optical machine-readable labels attached to items that record information related to the item. It was initially patented; however, its patent holder has chosen not to exercise those rights. Recently, the QR Code system has become popular outside the automotive industry due to its fast readability and greater storage capacity compared to standard UPC barcodes. The code consists of black modules (square dots) arranged in a square grid on a white background. The information encoded may be made up of four standardized types (“modes”) of data (numeric, alphanumeric, byte / binary, Kanji) or, through supported extensions, virtually any type of data.

A QR code is read by an imaging device, such as a camera, and formatted algorithmically by underlying software using Reed-Solomon error correction until the image can be appropriately interpreted. Data is then extracted from patterns present in both horizontal and vertical components of the image. As a variety of industries utilize the QR code today, the applications for use can vary from product tracking, item identification, time tracking, document management and general marketing purposes (Wikipedia)

So in essence the main use for QR codes is to make life easier for you – the user. No more long-url typing on a small keyboard and a mini input window (I get so frustrated with this sometimes). Just hold the phones camera up to the code and whooop there is your page.
But few know that there is much more you can do with a QR code.
You can (after scanning):

Send a Text Message
Send an Email
Display Contact Data
Send an Event to Calendar
Direct the user to a Phone Number
Give WiFi access
Display a YouTube Video
Display a Google Maps Location
Link to Social Media
Download an APP
Display an iTunes Link
Dropbox Location
Start a Skype Call
Link to Paypal

Just let your imagination run wild with all the possibilities.
QR codes are also a great mixing tool with traditional marketing. Think about all the saved space on an expensive magazine ad.
Put one or more on a flyer.
Stick one on your car, van or truck.
Display them in your shop window
etc. etc. etc.

Here are some really good QR apps for your phone: