They initiate an action on your phone (text is displayed, music is played, a map appears, video launches, you land on a web page, etc.) Scan a specific checkerboard pattern and a smartphone will react in a specific way.
What are your goals and objectives? How does using a QR barcode fit in? Be clear so you can implement with focus. Be clear so you can measure against your goals and objectives before, during, and after. Be clear so you can sell it to your boss/client/partners.
They need to deliver a mobile experience. Phones have limitations but your smartphones is always online, has a rich browser, is location aware, can deliver video and audio, is used to share information with others, etc. And QR barcodes can trigger these actions.
The only way to know if you've reached your goals, stayed within your budget, and had an impact on the world is to measure against your goals and objectives. You've defined them, you've implemented coherently with them, so build in data collection and analytics so you can quantify your efforts. If you're selling something, can you track transactions driven by QR? You do this for other projects, online and otherwise, so do it for your QR projects too. It will shorten your learning curve and arm you with personal experience for future efforts.
Take QR seriously. They are well established in Japan as well as Europe. Americans are late to the party. Plan well but don't implement half-heartedly. Be creative but never forget to educate your audience until QR becomes mainstream. You may well be someone's first encounter with QR and the mobile web. Make a good first impression.