Hahaha, I inteded to keep minions under the lid until after the game to keep the newbies on the team guessing but seems the cat is out of the bag now.
Okay, so what are Minions? Well like slave said minions are monsters that die very, very easily. In fact they have just 1 hp. And they deal static (i.e. no roll) damage which is considerably lower than what you'd expect from a normal monster of that level. So are they just chaff to be swept? Not... exactly. You see for all their weakness minions have still have defense and to-hit numbers equal to those of non-minion monsters, and they come in number of four to five for each normal mob they replace in the encounter so they can and will swarm you if given chance. And that's why you want to have a controller in the team. As for minion status being "obvious" it's a complex thing. On one hand I am not going to come out and point "this mob is a minion" but it's a) kind of obvious when one folds like a card tree after a single hit and b) if I remember and can fit it into description I'll try to clue in that some monsters seem to be "weaker" (couldn't really fit it this time around). Well that and if something arrives in numbers of 5 and above it's a minion suspect as well.
Design-wise minions are there to do three things: a) add some bodies so the conflict feels "bigger", b) make you feel boss when you mow them down left, right and center and c) represent "stronger" monsters you've met that are now beyond your notice because you outlevel them so much (those "stronger" skeletons you fight now? In five or six levels a skeleton minion relative to your level could represent them. They're just not a threat at this point.)
Edit: Oh, managed to forget - minions also never get hurt on a missed attack, even if it otherwise would deal damage.