Three tier architecture modularize application in 3 parts.
1. User Interface (Client) Layer
2. Middle layer
3. Data Access Layer
1. User Interface (Client) Layer: This is the primary layer of application, by which user interacts to application. We also call is UI.
2. Middle layer: This is Middle Layer, Where most of the business logic and rules are executed. It also provide functions such as queuing, application execution, and database staging etc.
3. Data Access Layer: This is also called by the famous acronym "DAL" component. It has mainly the SQL queries which do the database operation part of the job.