QR codes are square barcodes that contain information that can be retrieved by smartphones. For instance, this QR code contains a URL. Scanning this code will provide the link to the smartphone owner, who will then not need to type the full address on his device to access the information.
QR codes can also contain contain email addresses, phone numbers, SMS messages, or plain text.
These codes can be used in real spaces (printed out on posters, for instance) or on-screen.
An analogy: QR codes could as well be the short URLs of the physical space.