What are directives in Angular?

Directives are classes that add behavior to DOM elements or transform DOM layout. Three types: (1) Component directives: components ARE directives with a template (the most common type); (2) Structural directives: change the DOM structure by adding, removing, or manipulating elements. Prefixed with * (syntactic sugar for <ng-template>). Built-in: *ngIf: <div *ngIf="isLoggedIn; else loginTemplate"> — conditionally includes/removes an element; *ngFor: <li *ngFor="let item of items; let i = index; trackBy: trackById"> — repeats an element for each item in a list; *ngSwitch: conditionally display one of several views. New syntax (Angular 17+): @if, @for, @switch control flow; (3) Attribute directives: change the appearance or behavior of an existing element. Built-in: ngClass: [ngClass]="{ active: isActive, error: hasError }"; ngStyle: [ngStyle]="{ color: textColor, fontSize: fontSize + 'px' }"; ngModel: two-way binding; Custom attribute directive example: create a directive that turns text yellow on hover. Creating a directive: ng generate directive highlight — class with @Directive({ selector: '[appHighlight]' }) decorator. Access the host element via ElementRef and Renderer2 (DOM manipulation).