Data Access layer is actually a part of Architecture layer. It has 2 tier,3 tier or N tier Layer. Generally we use 3 tier Layer 1) Presentation layer,Business Logic layer and then Data Access Layer. Data Access layer is a medium to talk between database and Business Logic layer.

It helps to maintain flexibility,resuablity and even secuity also. In security SQL Injection can be stopped with 3 iter Archietcture.