Zend Framework is a PHP framework, written and maintained by Zend. It is designed to make development of web applications secure, streamlined and reliable.
As with all frameworks there is a learning curve with Zend. However, as soon as you get over this it gets a lot easier. I found that the best way to understand how to program with the Zend Framework is to find some tutorials. Luckily, there are people out there who have created a few Zend tutorials.
Rob Allen has a nice Zend Framework Tutorial which is available at his site. Rob also has a nice tutorial on Zend Framework authentication.