Catalyst is a release-building tool, designed to automate complex tasks normally associated with the building and maintaining of releases for open source projects, specifically distributions of Linux. Currently, it only supports the Portage package manager. Work is underway to support additional package managers.
Catalyst is based on a project called stager, which was written by Daniel Robbins. The intitial catalyst code was written by Daniel Robbins (as stager) and John P. Davis. Starting with catalyst 1.1.1, Chris Gianelloni took over as the primary maintainer of catalyst and has continued to maintain the project since.
For more information on catalyst, especially with regards to its usage within Gentoo, look at http://www.gentoo.org/proj/en/releng/catalyst/index.xml.