Zephr is, primarily, a decision engine. It is designed to make granular, high-scale decisions about which parts of your content a visitor can see – or how that content should be manipulated.
Typically, the Zephr CDN is used to implement the Decision Engine. That is, Zephr operates as a reverse proxy to your content (which could be HTML, for example) and will manipulate the content as it passes through, based upon the results from the Decision Engine.
Zephr also comes with a customizable identity management module (IDM), which allows registration, authentication, custom user schemas, corporate accounts and a bunch of other features. The IDM works seamlessly with the Decision Engine, allowing identity-based decisions out of the box.