More precisely referred to as "action drafting", this mechanism requires players to draft individual actions from a set that is available to all players. Players generally draft actions one-at-a-time and in turn order. If the game is structured in rounds, then all actions are usually refreshed so that they become available again for drafting. There is usually(*) a limit on the number of times a single action may be drafted in the same way for the same price. 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 another player. As such, not all actions can be taken by all players in a given round, and action 'blocking' occurs.
Actions are commonly drafted by the placement of game pieces or tokens on the selected actions. Each player usually has a limited number of pieces with which to participate in the process. Some games achieve the same effect in reverse: the turn begins with action spaces filled by markers, which are claimed by players for some cost.
From a thematic standpoint, the game pieces which players use to select actions often represent workers of any given trade (this category of mechanism, however, is not necessarily limited to or by this thematic representation). In other words, players often thematically "place workers" to show which actions have been drafted by individual players. For example, in Agricola 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 a piece on a given space, that action is no longer available until the next round.
Keydom, which was published in 1998, is widely recognized as the first of the worker placement genre of games. Early design experiments with the mechanism include Way Out West (2000) and Bus (1999). Well known examples of worker placement include Agricola (2007), Caylus (2005) and Stone Age (2008).
(*)The use of the word "usually" in this context is a somewhat controversial point of discussion. There are many who feel that 'action blocking' is a defining element of worker placement. In that case, there must always be a limit on the number of times a single action may be drafted each round.
Currently, the oldest dated and tagged game is Silverton (1991), although the included tag is controversial since the mechanisms do not meet many of the accepted criteria noted above.