title: what's the difference between service layer and domain model layer
body: <p>for example, I have a user table, to be layer-ing, I create such POJOs:</p>
  
  <p>UserEntity.java <br/>
  UserDao.java <br/>
  UserBO.java (business object, domain model?) <br/>
  UserService.java (for service layer) <br/></p>
  
  <p>what's the difference between UserBO.java and UserService.java? why we seperate it to two objects?</p>