Hamstring is an Arms Warrior ability learned at level 32. It is probably the most important PvP ability a Warrior has, slowing the target's movement speed. People are substantially easier to catch when they can't run quickly. This ability is also useful in PvE when fighting humanoids, as they run away when their health is low. The ability is relatively inexpensive, and in a PvP encounter is probably the ability you want to use right after your initial charge.
Hamstring is a 10 Rage skill that slows an enemy target 50%. The snare effect lasts 15 seconds and has a very low cooldown, so in a PvP battle you should make sure hamstring is constantly applied, as competent PvP players move around a lot.
Helpful in a variety of situations, this skill is one of the warrior's few forms of crowd control, primarily used for slowing enemies down in preparation to them becoming runners. In an ideal fight this should be applied somewhere around 40% to 30% health of any enemy you know will try to run and get help - mostly humanoids.
Its downside is that it requires a melee hit, so if they are already running, you will be unlikely to slow then down with this. It also only affects one target, making it difficult to use as a way to escape bad fights, since each individual enemy will need one Hamstring each.
In PvP this is a very useful skill, if used in conjunction with Charge on any enemy who overextends beyond his line of troops. Charging, Hamstringing, and then breaking off back to safety will leave them exposed and unable to run for cover effectively, allowing your team's hunters and mages to finish the enemy off.