Tank
In MMO parlance, to 'tank' is to keep the enemy focused on a class which is the most durable. In LOTRO, Guardians and Wardens are the primary tanking classes. Guardian being the only class able to use both Heavy Armour and Heavy Shields by default, and Wardens (they do wear medium armor, but are equally capable as a Guardian) due to their high evade, parry and block ratings and powerful self heals.
The Champion class is also available to tank, to a limited extent, if needed.