hi rajesh79
you can get information about this theoretical information on any .net site. Only make a search on google. But the actually the difference is that the transaction is taken in three steps. We generally use namespace, class , property in this.
When any transaction is taking place at that time we use get and set propers and pass values.
i know i can not explain. May be you are more confused. so..
http://www.beansoftware.com/ASP.NET-Tutorials/Three-Tier-Architecture.aspx
see this link. This will make you more clear what i am trying to say. As per my view here the theoretical and practical implementation is different. Best of Luck...