Chain stunning or stunlocking is the tactic of hitting a foe with an overlapping series of stun abilities, to the point that it can take few or no offensive actions while the party deals damage to it freely.
By its very nature, this tactic is useless against foes that cannot be stunned (either because they are immune, or because their levels are above the allowed level caps of the stun abilities available to the party), but its ability to lock down foes who can be stunned is incredibly potent, making it a go-to tactic for easing the burdens of tanking and healing when facing stunnable foes.
From the middle levels onward, Paladins can (largely) stunlock a single enemy all by themselves, assuming the stuns are not resisted, making them an ideal tank for a party intending on chaining stuns together.
Enchanters can apply both single target and AoE stuns, mixing and matching them to great effect.
Berserkers can participate in a chain-stunning rotation without interfering with their killing efficiency, making them an ideal pure melee DPS class to have in a party using this tactic.
Magicians can use an air pet to apply intermittent stuns, adding to the potential lockdown of a party, while only modestly hurting the magician's overall DPS compared to using a more offensive pet like water or fire. Beastlords can also inflict stuns as a random proc from their warders.
Wizards and Clerics can also add stuns of their own to a chain-stunning rotation, but they must do it at the opportunity-cost of not casting some other spell. In the case of wizards, this means doing a bit less DPS, and in the case of clerics, this usually means doing a bit less healing. (Though in the cleric's case, the healing may be unnecessary in the first place if the party is delivering enough stuns with the right timing.)
Monks can gain minor stunning utility by investing in the Stunning Strike AA.
At the highest levels.of play, druids also possess limited stunning abilities.
The crippling blow criticals triggered by warriors who are in a berserker frenzy can stun mobs of level 55 or below, which can allow a warrior at low health to single-handedly chain-stun such a foe if Mighty Strike Discipline is active.
Bards, shamans, rangers, rogues, necromancers, shadow knights, and warriors (except in the fringe situation described above) lack the ability to contribute much to a chain-stunning effort, though they can of course help with pulling or killing the foes in question.