You can generate a QR code pointing to a website, to a Google Maps location, with plain text, linking to a Facebook/Twitter page or to directly update a social network by just scanning the code. Type the relevant information in the corresponding box on the website and it will generate a QR code for free.
To make a QR code that looks a little out of the ordinary, you can change the foreground/background colour and add a photograph or logo into the code. To read QR codes, you need a QR code scanner app on your smartphone. Google Goggles and NeoReader are free apps that work great on Android, Windows Phone and iOS devices.
If you have a BlackBerry, check out QR Code Scanner Pro. All that you need to do is launch the app, point the phone's camera towards the code and it will automatically show you the contents. Make sure that your phone has a working data connection (Wi-Fi or cellular) in order to view or act on the information provided.