Root access from a Shell User
 

I have been reading this so many times on the internet that if some has a shell user login to My Box then there is a high possibility of getting root access from there.I really don't understand how can this be possible.Let suppose the user connects thru ssh to the box remotely.The how is the user going to know my root password?
can some one educate.

Some hacker can exploit buffer overflow vulnerabilities.

That's IMHO a minor issue. It's important to disable root access with ssh and use either secure passwords or just disable password access with ssh. I have enabled ssh access from the internet to my box at home (I'm accessing my box at home from my office from time to time) with ssh keys only.