What is 3-Tier architecture ?

1. Presentation Tier or Web Server: User Interface, displaying/ accepting data/ input to/ from the user 
2. Application Logic/ Business Logic/ Transaction Tier or Application Server: Data validation, acceptability check before being added to the database and all other business/ application specific operations
3. Data Tier or Database server: Simple reading and writing method to database or any other storage, connection, command, stored procedures etc