Domain Service :
Encapsulates business logic that doesn't naturally fit within a domain object, and are NOT typical CRUD operations - those would belong to a Repository.
* Domain Services along with your Domain Objects is sensible
Application Service :
Used by external consumers to talk to your system (think Web Services). If consumers need access to CRUD operations, they would be exposed here.
* Application Services will typically use both Domain Services and Repositories to deal with external requests.
Infrastructure Service:
Used to abstract technical concerns (e.g. MSMQ, email provider, etc)
