Nearly all fantasy based mmorpgs like World of Warcraft and Warhammer Online have a class that is most commonly known as a tank. For example, in wow this would be the warrior. These are classes that are mainly invited to the group to absorb damage, rather than deal damage.
Since the point of the tank is to absorb damage, then in addition to heavy armor that a tank wears, a shield is very important. It will deflect a lot of damage by increasing your defense rating. Also a shield in most mmorpgs will usually have extra stats that increase health points allowing the tank more survivability.
The tank class will be the one taking all the damage from the main mob that the group is attacking, so a healer in the group is essential. In addition, your party will require a damage dealing class so that mobs die a lot quicker, since the tank is there to take damage and not deal it. For encounters where multiple enemies are attacking, a crowd control class is essential since you should not be expected to keep all the mobs on yourself. Examples of crowd control class would be the Frost Mage in wow.
It is important that your group has an understanding of agro management. What this means is that mobs stay focused on you and not attack other members. Mobs usually attack other party members for three main reasons. The first reason is that you are not using your abilities to force the mob to concentrate on you. One skill to use and that is common across most mmorpgs is taunt. This skill will makes the mob hate you the most, and you will remain the focus of its attacks.
Another reason why a mob attacks another player is when the player has done too much damage too quickly, causing them to be a bigger treat that you. Damage dealing classes need to spread out their damage evenly, and all deal out burst damage towards the end of the encounter.
Badly played healers can also inadvertently peel away mobs from you, if they suddenly heal you for a high amount. The best strategy for healers, is to heal for small amounts, and only do a big heal in emergencies. Even better would be if the healer has a heal over time type ability, which they cast on you once, but will continue to heal you every few seconds for a small amount of heal.
So being a successful tank requires good equipment, but also joining a well balanced party who understands the mechanics of agro management.