Portage is the heart of Gentoo Linux, and performs many key functions. For one, Portage is the software distribution system for Gentoo Linux. To get the latest software for Gentoo Linux,
you type one command: emerge --sync. This command tells Portage to update your local "Portage tree" over the Internet. Local Portage tree contains a complete collection of scripts(ebuild scripts) that can be used by Portage to create and install the latest Gentoo packages. Currently, gentoo have more than 10000 packages in Portage tree, with updates and new ones being added all the time.
Portage is also a package building and installation system. When one wants to install a package, just type emerge packagename, at which point Portage automatically builds a custom version of the package with exact specifications, optimizing it for the hardware and ensuring that the optional features in the package that one wants are enabled -- and those one don't want aren't.
Portage also keeps one's system up-to-date. Typing emerge -u world -- one command -- will ensure that all the packages that one wants on his system are updated automatically.
cheers, make world open.