I am quite new to Linux administration so I am having trouble understanding yum update and grub boot list. 

I recently did a **yum update** on an old CentOS machine. Everything is good except there is multiple boot options appear in grub boot list and I wonder why? I managed to do a Google search and figured I can manually configure the boot order in /etc/grub.conf.

1. Does it mean I now have multiple OS installed?
2. Will my grub list grow if I do future yum update?
3. Do I need to clean up old item from the list?

Thanks.