A directive in Angular is a class that adds behavior to an existing HTML element, or extends the behavior of an HTML element by creating a new one. Directives can be used to manipulate the DOM, to add dynamic behavior to a web page, or to create custom HTML elements.
There are two types of directives in Angular: structural and attribute.
1. Structural Directives: Structural directives are used to change the structure of the DOM by adding, removing, or manipulating elements. Examples of structural directives are `ngIf`, `ngFor`, and `ngSwitch`.
2. Attribute Directives: Attribute directives are used to change the appearance or behavior of an existing element. Examples of attribute directives are `ngClass`, `ngStyle`, and `ngModel`.
In Angular, you can create your own custom directives to extend the functionality of your application. This can be useful if you want to reuse common behavior across multiple components or if you want to create a new HTML element with custom behavior.
In summary, a directive in Angular is a class that adds behavior to an existing HTML element, or extends the behavior of an HTML element by creating a new one. There are two types of directives in Angular: structural and attribute, and you can create your own custom directives to extend the functionality of your application.