Acronym for YellowDog Updater, Modified.

Yum is an "automatic updater and package installer/remover for RPM systems. It automatically computes dependencies and figures out what things should occur to install packages. It makes it easier to maintain groups of machines without having to manually update each one using rpm."

Pros

Small updates (single file headers, only updated when changed), designed especially for RPM systems

Cons

Slow updates caused by of lots of small files, useless on other distributions

It was written for YellowDog Linux for the PowerPC, but it has taken on a following since being adapted for RedHat and adopted as the official package distribution system for FedoraCore (replacing the RedHat Network.)

Compare with APT.

All the main FedoraRepositories support Yum; RedHat staff and other distributors who want to easily make packages available tend to use it instead of apt.