Automated mob farms?

What does that mean exactly? Does that mean all forms of  xp farms or what? Like if I find a mob spawner just destroy it since it cant be used?

No. As far as I understand, you cannot set up a redstone contraption that automatically kills the monsters and lets you collect the drops.

I think many servers have issues with mob spawners being on all the time, as they can cause lag. The last server I played on allowed automated mob farms, but they had to be able to be switched on and off, and were required to be switched off when not in use.

What Elijah says is true. You can sit in a dungeon all day long or a dark room and kill mobs however long you want. However, you cannot create a contraption that just brings mobs to you to kill or even kills them for you. There are two main reasons for why this is, and Elijah highlighted the first issue. The problem with mob farms is that they cause lag on the server because you are generating mass amounts of mobs. I understand this can be circumvented with an on/off switch however, this brings me to my second point. I believe, and Jeb himself even said, that it's not an intended part of the game. In my opinion they are exploits. You are gaining something for doing nothing, and that's not cool from a server admin's stand point. I hope this clears some things up.

Okays, cause I found a spawner and I just stand in a hole and wait for a mob to come to me and bash it,  so, if thats wrong I'll just destroy the spawner.

No you're fine, just as long as it's not automated or anything like collecting the mobs in one place with water and such.