Here are a few starters commands, these are the most common.
yum check-update (check to see if any packages need updating)
yum update (updates any/all packages that need it) yum update packagename1 packagename2 (only update packagename1 & 2)
yum install packagename1 (installs packagename) yum install packagename (same as above but does not prompt you for confirmation)
yum list available | grep -i packagename (checks to see if packagename is available at yum server)
yum search packagename (useful if you're not sure the name of the package, ie. httpd vs apache, RedHat names the Apache package httpd)
yum info packagename (similar to rpm -qi, give you info on packagename)
yum upgrade (I've only used this once, upgrading a test system running RHEL 3ES to CentOS, seems like if you point your Yum conf to a different distro's yum repo it will upgrade to that version, probably also useful for upgrade from CentOS2 to CentOS3)
For install, update, and upgrade, you can pass it the -y variable and it won't prompt you to confirm your actions. You can also customize your yum.conf to exclude certain packages from update/upgrade like the kernel. You can also specify that some packages(again kernel) only get installed and not upgraded.