The most powerful part about aggregates is that they dispatch Domain Events which can be used to co-locate business logic in the appropriate subdomain.

Domain events are simply objects that define some sort of event that occurs in the domain that domain experts care about.

Typically when we're dealing with CRUD apps, we add new domain logic that we've identified by adding more if/else statements.

However, in complex applications that can become very cumbersome (think Gitlab or Netflix).

Using Domain Events, instead of adding more and more if/else blocks

https://stackoverflow.com/questions/2268699/domain-driven-design-domain-service-application-service

Services come in 3 flavours: Domain Services, Application Services, and Infrastructure Services.

• Application Services : Used by external consumers to talk to your system (think Web Services). If consumers need access to CRUD operations, they would be exposed here.
• Infrastructure Services : Used to abstract technical concerns (e.g. MSMQ, email provider, etc).
• Domain Services : Encapsulates business logic that doesn't naturally fit within a domain object, and are NOT typical CRUD operations – those would belong to a Repository.

Keeping Domain Services along with your Domain Objects is sensible – they are all focused on domain logic. And yes, you can inject Repositories into your Services.

Application Services will typically use both Domain Services and Repositories to deal with external requests.

The differences between a domain service and an application services are subtle but critical:

• Domain services are very granular where as application services are a facade purposed with providing an API.
• Domain services contain domain logic that can’t naturally be placed in an entity or value object whereas application services orchestrate the execution of domain logic and don’t themselves implement any domain logic.
• Domain service methods can have other domain elements as operands and return values whereas application services operate upon trivial operands such as identity values and primitive data structures.