An 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."

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

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.

Some users find that it does not work as well as advertised; see f.ex. Fedora and the repository politics. Of course, YMMV.

There is now a nice front end for Yum if you use FedoraCore 4 or higher called yumex. To install just type

yum install yumex

Also a note if upgrading from FedoraCore 3 to 4 and your yum breaks go back to a default yum.conf file as most repositories do not yet work for FedoraCore 4.

Overview of Yum commands

Taken from the Fedora Wiki.

List all available packages:
yum list
See if there are updated packages available:
yum check-update
# or
yum list updates
Update all installed packages that have a newer version available:
yum update
Install specific packages and their dependencies:
yum install package(s)
Search all known packages entries (descriptions etc) for word:
yum search word
Show basic information about a package:
yum info package

All of the above commands can take shell-style wildcards (*,?) instead of a package name. Check the yum man page for more details.