There's a monk forum further down the list. You should be able to find plenty of details in the threads in there:
http://www.guildwarsguru.com/forum/f...play.php?f=207
Basically, a 55 monk is a monk that has applied Sup. runes (no vigors) to his armor and uses a -50hp offhand item to get his total health down to 55 HP.
Then they bring Protective Spirit which makes attacks on you only do damage equal to 10% of your health. If your health is only 55, and because of how GW rounds numbers, then each attack only does 5 HP of damage. If you then also bring skills that increase your health regeneration enough, then you become nearly impossible to kill.
It's a very popular farming build. Not used often in PVP except to cause grief to people in Random Arenas.