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In Angular, a directive is a class that extends the behavior of elements in the DOM. Directives can be used to manipulate the appearance or behavior of DOM elements or to create reusable components. There are three main types of directives: structural directives (e.g., `*ngIf`, `*ngFor`) that change the DOM layout, attribute directives that modify the behavior or appearance of elements, and custom directives that developers create for specific needs. Directives play a crucial role in enhancing the functionality and flexibility of Angular applications by providing ways to encapsulate and reuse code.

`HttpInterceptor` is an interface in Angular that allows you to intercept and modify HTTP requests and responses. By implementing `HttpInterceptor`, you can add custom logic to the request or response pipeline, such as adding authorization headers, logging request details, or handling errors globally. Interceptors are registered with the `HttpClientModule` and can be used to provide cross-cutting concerns that affect multiple HTTP operations throughout the application. This approach helps maintain a clean and consistent handling of HTTP communications.