Tanking is the act of taking damage so that other players do not have to or handling damage. Tanks are usually "pure" warriors who are trained highly in melee skills (Attack, Strength, Defence, Constitution) and/or long-range skills (Ranged, Magic). They favour Defence and usually have high Prayer levels too. Tanks are jokingly referred to as "human meatshields" because in some circumstances, they're simply doing nothing but taking damage and healing when necessary. Tanking occurs in many places, such as Clan Wars and other combat related activities. The earlier examples of tanking would be in the old multi-combat Wilderness, and when clans clashed, Tanks would head in first and "Tank" damage, some of the more recent examples of tanking are "Bandos Tanks" who take all the heavy damage from General Graardor, allowing the other team members to deal the damage back.
Tanking, in the bossing term, generally means taking the heavier, main damage from an enemy, be it monster or other players, though it does not necessarily mean that they take all the damage. An example would be a team fighting General Graardor in the Bandos' Stronghold of the God Wars Dungeon. General Graardor routinely uses 2 attacks, a powerful melee attack that hits one person, often dealing 500+ damage, and a slightly less powerful Area of Effect Ranged attack, which is dealt to every person in the vicinity. In this case, the tank would use the Protect from Melee prayer (or the Deflect Melee curse) to render General Graardor's threatening Melee attack less harmful (as General Graardor only uses his Melee attack on one player, and does not switch to another player until his target is dead) leaving only the Ranged attack to be worried about. All the teammates of the tank would use the prayer Protect from Missiles (or the Deflect Missiles curse) so that they receive absolutely no damage whatsoever from General Graardor. However, the flaw is that the tank who must use the prayer "Protect From Melee" must constantly heal because he/she is vulnerable to the Ranged attacks. As in many situations, the critical role is the role of the tank.
The tank is often the single most important player in a team launching an assault on a boss monster such as General Graardor, and if the tank fails to do their job adequately, tragic events for the rest of the team members are likely to follow. As an example, K'ril Tsutsaroth, the Demon in Zamorak's Fortress of the God Wars Dungeon uses two attacks, a powerful Melee attack and a much weaker magic attack. Usually, the tank prays against the Melee attack, while the others pray against the magic attack. If the designated tank dies, then the other team members are susceptible to K'ril Tsutsaroth's melee attack, which often kills in as little as 2 blows. The tank is the only person who takes the heavy damage in this situation, and generally speaking, if the tank dies/teleports and no other person can quickly enough assume the position of the tank, then the entire team will likely die or cease to continue.