How is business logic written for a project?

You can create a seperate class library project for business logic layer. There are lot design pattern also available for .net.

Business Logic :
http://www.dotnetjohn.com/articles.aspx?articleid=260
http://msdn.microsoft.com/en-us/library/aa581779.aspx
http://msdn.microsoft.com/en-us/library/bb470374.aspx

Design Pattern :
http://www.dofactory.com/Patterns/Patterns.aspx
http://www.designpatternsfor.net/

Hope this helps you.