How does a 3 tier application differ from a 2 tier one?

Tiers are the physical units of separation or deployment, while layers are the logical units of separation.

Imagine that you’re designing an e-commerce website. A 3 tier architecture would consist of web pages, a web server and a database, with the corresponding 3 layers being the “Presentation”, “Business Logic” and “Database” layers.
If you take the database tier and layer out then your have a 2 tier architecture.