Zend Technologies Ltd. is a company started by Zeev Suraski and Andi Gutmans. The product of theirs you most likely heard of is Zend Framework, a PHP framework. It's however not their only product. Another one is the Zend Engine, which is a PHP interpeter and the virtual machine at the heart of PHP and the reason there are references to Zend all over the place.

Zend developed the Zend Engine, a virtual machine. PHP4 is based on the Zend engine. Zend is more than just the framework, it literally is the core of PHP.