Gentoo is a computer cross-architecture Operating System based on Linux Kernel, and uses the Portage package management system. Gentoo OS can be installed on x86 (32 & 64), Power PC (32 & 64), Sparc (32 & 64), ARM, Cell BE, and others. Gentoo used to be known as Enoch.
The Portage package management system implements Sun's "Write Once, Run Anywhere" motto. Portage packages are written in Python and Bash, and are not pre-compiled. They compile during installation, making the installed program architecture native. That means fast processor and at least 512 megs of memory, to get reasonable compile times. Otherwise your installations will last very long and bring your system to a crawl during the Installation.
It should however be noted that you can set up the installation to compile the packages for another computer, allowing you to use extremely old hardware connected to your computer network. A similar approach is used to install applications for Gentoo on slower hardware like Raspberry Pi.