In a normal round, you can perform a standard action and a move action, or you can perform a full-round action. You can also perform one or more free actions. You can always take a move action in place of a standard action.
The “Action Economy” is the system that limits what each playIer can accomplish in one turn. In the d20 SRD the rule cited above compactly summarizes the action economy in the d20 system (i.e. D&D 3.5). Each turn a character can take either (1) a full-round action, or (2) both a standard action and a move action. In addition each player can take “free actions.” The action economy is not the choices you are presented with during your turn, rather it is the number of choices the game allows you to make each turn.
The Action Economy exists in one form or another in every game. In turn based combat games, like the d20 system, the game has to limit the number of attacks a character can make each turn, the number of spells that can be cast, the distance the character can move, etc. If the player could choose to have a character attack and cast a spell and move twice and cast another spell and the attack again – the game would quickly lose all semblance of balance.
Compare d20 system’s Action Economy against the Action Economy in blackjack, where each player (generally) gets one turn and is allowed to make as many “actions” as she wants – briefly in blackjack the goal is to have the cards dealt to the player total to a number as close as possible to 21, if you go over 21 you lose (called busting). Each player gets one round of actions in each hand dealt (turn) and the player has effectively two choices (skipping the nuances) stand on the cards dealt or taking another card from the dealer (called a “hit”). The player is allowed to take as many hits as she wants, but the round ends once she stands or busts out. In blackjack play is limited because eventually taking another “action” will cause the player to bust and, therefore, to lose. Blackjack limits the value of actions rather than the number of actions. In strategy games like D&D, more actions is almost always going to be better.
The Action Economy in the d20 system helps tell the story of combat – as characters get more powerful they gain additional actions through access to spells like haste. The decisions a player has to make – for example to do a full round attack action swinging two or more times with a weapon – takes up all the character’s time, depriving him of movement. Sometimes a character quickly hits once and then uses movement to dash away. The limited Action Economy sets up a story where combat is faced paced and frantic – with characters dashing back and forth getting an attack in here and there, or alternatively, standing toe to toe and chopping at each other with rapid cuts.
Unfortunately, because expanding a character’s Action Economy creates more choices and gives more opportunities to attack or move – to control the battlefield – spells and abilities that introduce additional actions are markedly powerful and disruptive to game play. For example, summon monster spells allow a spellcaster to bring another creature into combat under his control – thus doubling that character’s action economy.
In the d20 system summoned monsters were controlled through verbal directions – speech – a free action – and generally run by the player as additional characters under his control . That meant that a player could have a turn consisting of multiple moves, attacks, spells cast, etc. In 4e D&D summoned monsters were a bit more complicated, allowing the player to choose either to spend one of his character’s limited action types to control the summoned monster, or else the monster would do a prescribed action (usually with a costs to the caster). i.e. a summoned hell hound left to its own devices would target as many creatures as possible each turn with its flame breath attack – friend and foe alike – and deal some damage to the character that summoned it. 5e D&D turns back toward an expanded action economy – allowing summoned creatures to act on verbal (i.e. free) commands – but explicitly turns the summoned creatures over to the DM to control rather than the player.
No matter what game you play – as a rules lawyer, take the time to examine the Action Economy is one of the most basic elements of a rule system – literally defining what a player can do on their turn. It’s a great place to start any analysis of a rules system.