Piwik is web analytics software that runs as a web application.
It works similarly to popular analytics software such as Google Analytics; an important difference is that Piwik users typically install and manage their own Piwik installations, rather than using externally hosted ones; hence, considerations w.r.t. maintenance and privacy protection are markedly different.
Piwik is open source (GPL) and actively developed by an international team of developers. Written in PHP, using MySQL as its database backend, it will run on most webservers. Optional functionality is available in the form of plugins; developers can write their own plugins.