These actions take a short amount of time (a quick spell, concentrating to re-direct an energy beam, mentally controlling a dominated subject). These take a bonus action instead of a standard action, allowing you to do one of each in a turn. Not all actions available to you allow for a bonus action, so most of the time you probably won’t have a bonus action to use. You can only use one bonus action on your turn.
If you use your bonus action to cast a spell, you cannot also use your standard action to do so, too, though you can use it for something else. You can only cast one spell per turn, regardless of how long it takes to do so.
An action that takes a bonus action may also be performed as a standard action, but not vice versa.
Some actions are performed in response to another character’s action. These are typically not done on your turn, but you are allowed one reaction against an enemy’s action, in each round. Sometimes these are defensive spells (like Shield) that can be used against an enemy attack. Other times they are a reaction to a situation (like Feather Fall which you may cast if you start to fall from a high surface). If you use your reaction, you get it back at the beginning of your turn.
Attacks of Opportunity are the most common form of reaction. These are triggered when an opponent leaves a square that you threaten or stands up from prone. When this happens, if you have a melee weapon in your hand, you may use a reaction to make a free melee attack. This attack happens before your opponent completes his movement.
Free actions don’t take any time at all, though there may be limits to the number of free actions you can perform in a turn. Some common free actions are described below.
Dropping an item in your space or into an adjacent square is a free action.
Dropping to a prone position in your space is a free action.
In general, speaking is a free action that you can perform even when it isn’t your turn. Speaking more than few sentences is generally beyond the limit of a free action.
You can stop concentrating on an active spell as a free action.
You can cast a quickened spell (see the Quicken Spell technique) or any spell whose casting time is designated as a free action. Only one such spell can be cast in any round, and such spells don’t count toward your normal limit of one spell per round.