What is a service in Angular and when would you use one?

A service is a class with a specific purpose that provides functionality not tied to a view, so many components can share it. Use one for:

• Sharing data between components.
• Business logic that should not live in components.
• External communication, such as HTTP requests.
• Utility functions and helpers.
• State management.

import { Injectable } from '@angular/core';
@Injectable({
  providedIn: 'root'
})
export class DataService {
  private data: string[] = [];

  getData(): string[] {
    return this.data;
  }

  addData(item: string): void {
    this.data.push(item);
  }
}

providedIn: 'root' creates one shared instance for the whole app, and the build drops the service if nothing uses it. Components receive that instance through dependency injection. Newer Angular versions also offer @Service(), a shorter way to declare the same root-provided singleton.