A tier is a row or layer in a series of similarly arranged objects. In computer programming, the parts of a program can be distributed among several tiers,
each located in a different computer in a network. Such a program is said to be tiered , multi-tier , or multi-tiered application.
The 3-tier application model is probably the most common way of organizing a program in a network. N-tier applications (programs) are those that are tiered but the number of tiers isn't specified or may vary.
Two-tier architecture is a client/server architecture , where a request to do some task is sent to the server and the server responds by performing the
task.Whereas a three-tier or a multi-tier architecture has client, server and database. Where the client request is sent to the server and the server
in turn sends the request to the database. The database sends back the information/data required to the server which in turn sends it to the client.