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What is a BackgroundService in ASP.NET Core, and what does it provide out of the box? How does graceful shutdown work for a BackgroundService, and what role does CancellationToken play? How should exceptions be handled inside a BackgroundService's ExecuteAsync loop to prevent the entire service from crashing silently? How does a BackgroundService differ from a Worker Service project template, and how do they relate? How do you register and run multiple independent BackgroundServices within the same application? How would you implement a scheduled (cron-like) background job in .NET without a third-party library? What is the significance of the order of AddHostedService registrations relative to StartAsync execution, and can you control it? How should a BackgroundService handle a fatal startup dependency failure, such as being unable to connect to a required message queue at launch? How do you safely access scoped services (like a DbContext) from within a BackgroundService, which itself is registered as a singleton? What is IHostedLifecycleService, and how does it provide more granular hooks than IHostedService's StartAsync/StopAsync?
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What is the difference between IHostedService and BackgroundService, and when would you implement IHostedService directly?

Beginner

Answer

IHostedService is the low-level interface with just StartAsync and StopAsync methods -- you implement it directly when you need precise control over startup/shutdown without an inherent long-running loop, such as a service that just registers something once and cleans up once. BackgroundService is a convenience base class implementing IHostedService specifically for the common case of a continuous work loop, handling the StartAsync/StopAsync plumbing for you so you only need to write ExecuteAsync.
// Direct IHostedService: no continuous loop, just start/stop hooks
public class MetricsRegistrationService : IHostedService {
    public Task StartAsync(CancellationToken ct) {
        MetricsRegistry.Register("my-service");
        return Task.CompletedTask;
    }
    public Task StopAsync(CancellationToken ct) {
        MetricsRegistry.Unregister("my-service");
        return Task.CompletedTask;
    }
}
Real-world example A service that simply registers the application with a service discovery system at startup and deregisters at shutdown implements IHostedService directly, since it has no ongoing loop -- unlike a queue processor, which extends BackgroundService instead.

Common follow-ups: Why does BackgroundService exist if IHostedService already covers this?;Can a class implement both patterns simultaneously?

Generic Host;Worker Services & IHostedService
All 15 Background Services questions