Rampage is probably the most interesting melee ability that monsters can have and also the most difficult one to understand. To make rampage even more complex, there is more than one type of rampage: single target rampage and AE rampage.
Usually, monsters have only one aggro list: the person who generates the most aggro is on top and when someone else generates more aggro than that person, the order of the aggro list changes. Monsters that rampage have an additional aggro list - called the rampage list or rampage order. A person makes it to the rampage list when they attack the monster. Barring two exceptions that will be covered in a moment, the order of the rampage list will not change once it is set; gaining more aggro won't help, moving out of range won't help, nothing else will help you to move up or down the rampage list and nothing will help you to be removed from the rampage list.
When single-target rampage goes off, the monster will perform a free round of attacks on the person that is on top of the rampage list. If the person of top on the rampage list is also the person who has aggro, then single target rampage will go to the person who is second on the rampage list. If the person to whom single-target rampage should go is out of range of rampage attacks (which is usually much higher than the range of that monster's melee attacks), the rampage will instead go to the person who is next on the rampage list. In other words, the topmost person on the rampage list, not including people outside rampage range and the person who has aggro, will always receive single target rampage.
Clearly, you want the single-target rampage to go to the person who can handle the hits and who will have the healing ready. Due to the fact that the rampage list can not change (again barring the two exceptions that will be mentioned in a moment), the person who should receive rampage damage needs to be the first person to aggro the monster - i.e. pull it. To ensure rampage does not go to casters or healers in case the rampage tank dies, more melees will usually ping an arrow to the monster right after the rampage tank aggroes it - so they will be right after the tank on the rampage list.
The other type of rampage is AE rampage. The mechanics are almost the same as with single-target mechanics, except that the rampaging monster will perform a free round of attacks on every single person who is on the rampage list and in range of the rampage attacks, with the exception of the person who is on top of the regular aggro list. This means that healers and casters need to stay out of rampage range (because they usually can not handle melee hits very well). Fortunately for the melees, AE rampage range is usually just slightly lower than the range from which melees can hit the monster, so AE rampage can very often be negated by attacking from the maximum possible range.
The exceptions that can remove a person from rampage list are bard Fade Memories and a successful memory wipe during feign death. Leaving the zone will also get you off the rampage list - but this can obviously not be used as a tactics to fight rampage.
Infuriation is another special monster ability closely tied to rampage. When a monster infuriates, all of its single-target rampage procs will change into AE rampage. Infuriation usually lasts only about 10 seconds, usually several times while the monster is alive. To negate the effects of infuriation, melee from the maximum possible range.