Portage is source-based package manager used by Gentoo and its descendants. It controls all process from fetching source through building it, installing into clean environment to "merging" with already installed software.
Ranked in these QuestionsQuestion Ranking
Pro Sandboxes build process
Portage uses a sandbox as a safety measure during build processes. This is done to ensure that no packages accidentally write outside a 'safe' location.
Pro Decide which dependencies to install
WIth portage you can decide and customize which dependencies to install through some thing called USE flags. These are keywords that when defined, will tell Portage that you want support for the chosen keyword.
Pro You can view a list of programs that can be installed
With portage you can view a list of all the programs that you can install by going to
/usr/portage and running
Pro Can install multiple versions of the same package simultaneously
Slotting is a feature which allows users to install multiple versions of a software simultaneously. This is especially useful for libraries which have changed interfaces between versions.
Pro Simple overlay management
Adding supplemental repositories, aka overlays, is easy with layman.
Pro Respects customized config files
By default, portage doesn't delete or move any customized config files, thus enabling competent users to modify any config file however they want.
Pro Allows both binary and source installation
With portage you can either compile packages from source or you can download and install their binary versions.
Con Very slow
Dependency resolution is very slow and single-threaded, so usually you will see one of your cores running like crazy for over a minute
Con High memory usage
Usually takes between 400-800MB of RSS (no problem to get over 1GB), so it's nothing for an old hardware.