Multi tier Architecture:

In software engineering, multi-tier architecture (often referred to as n-tier architecture) is a client-server architecture in which an application is executed by more than one distinct software agent. For example, an application that uses middleware to service data requests between a user and a database employs multi-tier architecture. The most widespread use of "multi-tier architecture" refers to three-tier architecture.
The 3-Tier architecture has the following 3-tiers:
1. Presentation Tier

This is the top most level of the application. The presentation tier displays information related to such services as browsing merchandise, purchasing, and shopping cart contents. It communicates with other tiers by outputting results to the browser/client tier and all other tiers in the network.
2. Application Tier/Logic Tier/Business Logic Tier

The logic tier is pulled out from the presentation tier and, as its own layer, it controls an application’s functionality by performing detailed processing
3. Data Tier

This tier consists of Database Servers. Here information is stored and retrieved. This tier keeps data neutral and independent from application servers or business logic. Giving data its own tier also improves scalability and performance.

In the Web development field, three-tier is often used to refer to Websites, commonly electronic commerce websites, which are built using three tiers:
1. A front end Web server serving static content
2. A middle dynamic content processing and generation level Application server, for example Java EE platform.
3. A back end Database, comprising both data sets and the Database management system or RDBMS software that manages and provides access to the data.