What separates one tier from another in the context of n-tiered architecture?

It depends on the application.

In a web application, for example, where tier 1 is a web-server, it may communicate with a tier 2 Application Server using RMI over IIOP, and subsequently tier 2 may communicate with tier 3 (data storage) using JDBC, etc.

Each of these tiers may be on separate physical machines or they may share the same box.

The important thing is the functionality at each tier.

• Tier 1 - Presentation - should be concerned mainly with display of user interfaces and/or data to the client browser or client system.
• Tier 2 - Application - should be concerned with business logic
• Tier 3+ - Storage/Enterprise Systems - should be focused on data persistence and/or communication with other Enterprise Systems.