Rammus is a jungler that excels at ganking through his high mobility and croud control. His jungle clear speed is sub-par to other junglers such as mundo or shyvana which farm much faster than rammus. Generally you need to get off good ganks to get ahead of the enemy jungler, if you are unable to do this you will generally fall behind in gold and farm.

Upon reaching level 3 you should look for a gank at either the top or mid lane. One thing that sets good rammus players aside from great rammus players is their knowledge of when and where to gank at the right time, while this is true for all champions in the jungle.

During the mid game you should do a mix of farming and ganking. This is very easy on rammus because of his high mobility with his power ball. Upon reaching max rank on your puncturing taunt your ganks are incredibly strong, so you should look to pick up kills in overextended lanes whenever possible. Another thing about rammus is, generally in solo queue people tend to forget the jungler exists and this is extremely beneficial to you, its just roll in to the lane and get a free kill, especially at lower ELO's.

Your role late game is to be a tank. You need to soak up a lot of damage in teamfights, lock down the enemy ad carry with your taunt and be as disruptive as possible. Your item build late game also offers a lot of utility to your team with shurelyas reverie and aegis of the legion.
Also you are very good at shutting down the enemy ad carry in teamfights with randuins omen and frozen hearts attackspeed slow.

For runes I run attackspeed reds, this allows for faster jungle clearing. I run flat armor yellows, this helps me survive the jungle and also synergies with my passive. I run flat magic resist blues, this is really just for survivability aginst mages and other sources of magic damage. And finally i run armor quints this the same as my armor yellows, jungle survivability and synergy between defensive ball curl and my passive

For summoner spells I take flash and smite. Flash is one of the best summoner spells in the game, the ability to instantly re position yourself is abo****ley invaluable. With flash you can escape dangerous situations or catch up to and enemy. In rammus' case you can flash taunt someone which is extremely surprising while ganking an enemy. I take smite as my other summoner spell. Smite is essetial when jungling with most junglers, one exception being cho'gath. Smite allows you to finish off the dragon or baron nashor so the enemy team cant steal it and also helps you with neutral monster buffs in the jungle.

Generally on rammus I start at wolves, then have a team mate pull the blue golem buff, after that I go to the wraith camp then to minigolems, go back to base and buy an extremely early philosophers stone. Then i go straight to red buff, then go gank.

So basically rammus is valued for his ganks as a jungler, he is extremely tanky and is very disruptive in teamfigts