"Gentoo is a free operating system based on either Linux or FreeBSD that can be automatically optimized and customized for just about any application or need. Extreme configurability, performance and a top-notch user and developer community are all hallmarks of the Gentoo experience.
Using a technology called Portage, Gentoo can become an ideal secure server, development workstation, professional desktop, gaming system, embedded solution or something else -- whatever you need it to be. Because of its near-unlimited adaptability, Gentoo is called a metadistribution.
Gentoo is also a community built around a distribution which is driven by more than 300 developers and thousands of users. The distribution project provides the means for the users to enjoy Gentoo: documentation, infrastructure (mailinglists, site, forums ...), release engineering, software porting, quality assurance, security followup, hardening and more." 
Please visit gentoo.org and support their distribution at gentoo.org (donation button in the upper right corner).