A library can be described as a collections of functionality that you can call.
A framework can be described as a pre-made program where in various places you can insert your own code. The framework then calls the code for you at the correct points.
In my many PHP applications, i’ve been using Zend more as a library, I am creating the “framework” and using what Zend has to offer as a library.
Is the Zend framework really a library?
It can be either or both. Zend is an MVC framework, however many of the components can be called independently of the framework itself.
For instance, I just read Agile Web Application Development with Yii1.1 and PHP5 and there is a chapter showing how to create an XML feed using the Zend Framework/Library. So my Yii framework was referencing Zend as a library. It has great objects and functions, so why not take advantage of them as a library when you can.