A mob trap is basically anything that messes with the mob other than your fist, sword, or bow. That includes suffocation, lava, and movement by water.

 

Apparently some new mob trap new also says they need to be in a 10x10 area which I do not find in the actual rules.

 

Is it a mob "trap" if it'd just a hallway where the mobs walk down, without the aid of water?

I would define a mobs trap as a structure that kills the mobs without player interaction, often attached to a spawning floor. So a 50 block drop or lava.

If you are hacking at there feet that is an xp farm to me because you are not trapping or killing the mob without player interaction. You still have to stand there for hours to get 50lvs of xp.

What about with cave spiders if you put an iron door between the two fence posts and hack at the spiders through the gap?

With out the water in this case.

I think a mob trap need to be better defined because mob trap is a loosely used term in Minecraft.

 

You've got water moving the mobs, which isn't allowed but if you have the doors like that, I don't know if that would be allowed or not because I don't know if you're supposed to be able to hit the mobs like that.

 

It was my understanding that the #1 rule was that the player must deal all the damage to the mob. and the #2 rule was that you could not artificially move a mod via water. Everything else would be exp farming. But I am not the authority on the subject.