A calling station, or station for short, is an extremely loose and passive player who plays too many hands and goes too far with them. He calls every bet, even with marginal hands and is not able to see when he's beaten. He also plays good hands passively, meaning: he rarely makes a bet to build up a pot or protect a vulnerable hand. They are named calling stations because the call is their favorite move. Hence, they are not bluffable.
Reasons for Playing Style
There may be many reasons why calling stations play this way. For one, they cannot tell the the strength of their hands and so they go too far with weak hands and do not play their strong hands aggressively enough. On the other hand, they could be looking more for the social aspect of poker; they just want to be involved in the play instead of creating an aggressive atmosphere. A player might also be a calling station because he lacks the confidence to do otherwise, seeing a bluff behind every opposing bet.
Maniac, TAG, LAG, Passive, Aggressive