This build is for a Support Tank/Jungler the idea of this build isn't to do damage but to take it all for your team (Like your suppose to).Ssome people often mistake Rammus For a tank that's suppose to get lots of kills this is wrong, its your job as a tank to assist your team in anyway possible and by absorbing as much damage as you can before getting out or in some cases Dying, Dying as a tank isn't a bad thing if your team wins because of it.

As a Support Tank pick up Ghost and Exhaust to have a an escape skill and an engage skill + the exhaust to assist by reducing the enemy's carry to make them an easy target.

When Jungling replace Ghost with Smite because smite is essential as a jungler. as a jungler it is still your job too assist the team in any way.

The item build gives utility and Tanky-ness in which Rammus needs.
For example the Randuins active effect will really help your team with chasing people down.
The Guardian Angel is IMO The best Item for a Tank gives you the Ability revive so you can tank for the Longest time possible.

The masteries and runes are your common tank masteries and runes for as much early game tanky-ness as possible.

Pro's
Extremely hard to kill end build
Great initiate
Can single out the enemies main threat
Randuins prevents enemies from running away
Revives himself
Con's
This build doesn't have a Thornmail like most other Rammus builds
Doesn't do all that much damage
if your team doesn't let you go in first your useless
If people don't understand your suppose to assist and not get kills you may be called a noob ignore this :)