QR Code short for Quick Response Code is a two-dimensional matrix bar code which is used to identify products. It was designed to have its contents decoded at a high speed.
Basic usage of QR codes: Text, Website URL, Telephone Number, SMS Message, Email Address, Email Message, Contact Details (VCARD), Event (VCALENDAR), Google Maps Location, Paypal Buy Now Link, YouTube Video, Wifi Login, Social Media.
To decode the information stored in the QR codes you will need to use the camera on your iDevice and one of the reader apps listed bellow.

1. Norton Snap QR Code Reader

Norton™ Labs Snap is the first QR code scanner that protects you, your mobile device, and your important Stuff from online threats by warning you of dangerous QR codes and blocking unsafe websites before they load on your device.
The free Norton™ Labs Snap app puts you one step ahead of potential threats and takes the gamble out of using QR codes.
Requirements: Compatible with iPhone, iPod touch (4th generation), iPad. Requires iOS 4.3 or later.

2. Microsoft Tag

The Microsoft Tag app lets you instantly connect to a whole new world of information and entertainment. This latest version now reads Microsoft Tag barcodes and QR Codes. No need to type long URLs or text short codes; simply scan a 2D barcode wherever you see one.
Requirements: Compatible with iPhone, iPod touch, and iPad. Requires iOS 4.0 or later.

3. Qrafter – QR Code Reader and Generator

Qrafter is a two-dimensional barcode scanner for iPhone, iPad and iPod Touch. Its main purpose is to scan and parse the contents of QR Codes. It can also generate QR Codes when you purchase the “Pro Pack”.
Qrafter is the first QR Code app that is designed for iPad instead of just taking the iPhone version and making it bigger.
Requirements: Compatible with iPhone, iPod touch, and iPad.Requires iOS 4.0 or later.

4. Quick Scan – QR Code Reader

Quick Scan helps you scan and decode all the QR codes around you, including contact, detail, plain text, website URL, telephone number, SMS message, email address, email message, calendar event, locations and more. It is transforming your life towards an easier and more convenient way.
Requirements: Compatible with iPhone, iPod touch (4th generation), iPad. Requires iOS 4.0 or later.

5. Paperlinks

With Paperlinks, scanning QR (“Quick Response”) codes is easy and fun. Your editable history will include items you’ve already scanned. And best of all, you can create your own QR codes at www.paperlinks.com, then have people scan your Paperlink code for an in-app (rather than web browser) experience.
Requirements: Compatible with iPhone, iPod touch, and iPad.Requires iOS 4.0 or later.

6. Scan 2 Link QR reader

Scan 2 Link mobile application is a free QR code scanner that helps you scan and organize your scans for future references. If you are already a Scan 2 Link web service user, you can login to you your accounts to get handy reports of your generated QR Codes for your microsite or campaigns.
Requirements: Compatible with iPhone, iPod touch, and iPad.Requires iOS 3.2 or later.

7. Qropit Social QR Code Reader

Qropit is the fun, social QR code reader. With Qropit you can share any QR codes you scan on Facebook and Twitter. You can also view what other users are scanning nearby!
Qropit supports video, photos, location, contact, websites and even coupons!
Requirements: Compatible with iPhone, iPod touch, and iPad.Requires iOS 4.3 or later.