How many types of Layers and what are those?

There are three layers

1.      Presentation layer or UI

2.      Business logic Layer

3.      Data Access Layer

1.      Presentation Layer  :  This layer should contain only code related to presentation.

2.      Business logic Layer :  All validations / Calculations related  to UI as well as Data Access layer should be performed using Business Layer.

3.      Data Access Layer :  All data source activities must be performed using data access layer. Data source doesn’t mean only Database’s. It also includes XML, strings, text files.