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.

• 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.