yum -y

[ValiantJakk]

What should should be updated with yum -y? I'm using CentOS 5 and I just want to make sure that I have an updated system.

[RobinVossen]

Nothing unless you want to say yes to all the next questions..

What the -y does is saying yes.
So if you want to update always without checking if you want to update you can do
yum -y update in crontab

If you do that there is a chance that your system changes without you wanting that.. Hope that helps