A multi-tier app consists of two or more programs normally running on
different computers, possibly in different locations. The most common
example (which some do not consider a real multi-tier app) is a
classic two tier client/server database app. Here you have a client
application running on one PC and a database server, such as InterBase
or SQL Server, running on another machine.
What most people think of when someone says "multi-tier" is a three
tier app. This typically consists of a client application, a middle
tier application server and a database server. All three tiers usually
run on different machines. The middle tier application server can
handle various tasks such as business rule enforcement and data
consolidation or distribution.