Gentoo's portage is similar to '
yum as in CentOS' or '
apt-get or aptitude as in Debian' or '
up2date as in RHEL'..
It downloads the sources off a mirror & compiles them for your system, automatically solving dependencies. Debian on the other hand has apt, which also solves dependencies for you, but uses binaries instead of sources.
In Gentoo, portage allow for gcc optimization flags & "use flags", both have an influence on your system & this flexibility makes Gentoo harder to troubleshoot.. These kinds of settings in Gentoo allow you to optimize your system to the extreme level, but if you're not carefull you could also break it seriously..
