A shill or bot is basically a fake or paid player that is used by the poker site in order to make the room look more popular. All of the major poker rooms claim that they do not use shills or bots at all on their site, "unlike some other poker sites". From my knowledge, only the really small sites have currently have any need for shills or bots, which you should avoid anyways. Many of the popular sites in the past however, employed props to make their site look more popular.

However, one tactic that poker sites use to inflate their numbers, is to include their play money players when they mention how many total players are currently on their site. In this regard, they can look twice as popular than they really are.