The horde framework is written in php, and provides the common tools a web application requires: classes for dealing with preferences, compression, browser detection, connection tracking, mime, and more.

the horde framework, by itself, does not provide any significant end user functionality; it provides a base for other applications and tools for developers. you will probably want to install some of the available horde applications, such as imp (a webmail client), or kronolith (a calendar).

there is a list of horde applications and projects at http://www.horde.org/projects.php.