The Zope Toolkit (ZTK) is a set of libraries intended for reuse by projects to develop web applications or web frameworks. It is developed by the contributors of the Zope Foundation.
The whole collection of ZTK libraries are used in various web frameworks and web application servers. Three of these are managed by the Zope project: BlueBream, Grok and Zope 2. If you install one of these systems, you will get a part of the ZTK along with it automatically.
The Release team is there to coordinate the release of new versions of the ZTK and make sure the interests of the three Zope projects are respected.
Note
The ZTK is not installable by itself directly, but its libraries are.