Generally, "backline" means a character that will only be interacting with other members of your team. Like you said, monks and BiP necros are examples of a backline character.
"Midline" refers to those who interact with both teammates and enemies, or who interact only with enemies but can do so from range. This usually includes mesmers, elementalists, offensive necros, some rangers and paragons. You get the idea.
"Frontline" is pretty obvious--teammates who frequently wade into enemy lines and act as the vanguard for the team. Warriors and dervishes are the obvious examples here.
Hope that helps a little.
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