Definition: ... a client–server architecture in which presentation, application processing, and data management functions are physically separated.
(wikipedia)
Possible tiers:
presentation tier(e.g. web server)
content)
Old Schoolclient-server where the client has logic and presentation.
Good Practice: Separate concerns and define (and use) application services even when you deploy them on the same server for convenience.
What about the Web browser or mobile apps?
smartplatforms