Learn writing class files and associate them with webform in ASP.NET(vb)

Question

Hi all, I am a entry-level developer working on ASP.NET since 6 months.I am generally fond of writing the code and whole functionality in code-behind of .aspx files.But my boss ordered me to use class files(ASP.NET(vb)) for code and just call those class files when necessary.I donno how to write them and when or where or for what and why to use them??wat all can we write in class files,can we write stored proc,giv sql connections!!I need some assistance.Internet is my only guidance!!Can any body help me regarding this!!plz feel free to fwd me any learning sites links,tutorials,samples if u hav any. U can mail me to [email protected] or post ur suggestions to this here