From the asp.net quick tutorial
A Simple Two-Tier Web Forms Page
The classic use for an external component is to perform data access. This
simplifies the code in your page, improving readability and separating your
user interface (UI) logic from the system functionality.
A Simple Three-Tier Web Forms Page
A three-tiered application model extends the two-tiered scenario to include
business rules between the UI and data access logic. This model allows UI
developers to work with a higher level of abstraction rather than directly
manipulating data through low-level data access component APIs. The middle
business component typically enforces business rules and ensures that the
relationships and primary key constraints of the database are honored.
If you have it installed, here is the link.
http://localhost/quickstart/aspplus/default.aspx?url=/quickstart/aspplus/doc/businessobjs.aspx