3 - Worker Placement is More precisely referred to as "action drafting", this mechanic requires players to draft individual actions from a set that is available to all players. In a given round, drafting is done one-at-a-time and in turn order until all players have had a chance to draft individual actions. There is a limit on the number of times a single action may be drafted each round. Once that limit is reached, an action can no longer be taken until a subsequent round or until the action space is no longer occupied by a worker. As such, not all actions can be taken by all players in a given round, and action 'blocking' occurs.
Usually, each player has a limited number of pieces with which to participate in the process. In other words, they "place workers" to show which actions have been drafted by which players. In Agricola, for example, you start with two family members who can be placed on action spaces to collect resources or take certain actions like building fences. When someone places on a given space, that action is no longer available until the next round.
Commonly, the used pieces thematically represent workers of any given trade, though this category of mechanic is not limited to or by this.
Well known examples of Worker Placement include Agricola, Caylus, Stone Age, Pillars of the Earth, Tribune, and Dominant Species. Early design experiments with the mechanic include Way Out West, Bus, and Keydom (though there is some disagreement about this).