Develop criteria in modular form

Hi, I am doing a project that have different specification such as search for the product type using a search tab panel. Afterwhich I can also download the product catalog and save to my desktop.
How do I make use of .net c#(Visual Studio 2008) to develop this codes in modular and link to the database(Mircrosoft server 2008)?
Can you guys give me any samples of tutorial cause I cant seem to find it online. Thanks!

If you want modular code, I would personally go with DLL (Dynamic Link Layer). It is one file that will house all of your classes. You can put the same DLL on your server and desktop computer. If you are looking at grabbing data from your web server, you might want to look into Web Services or WCF to bring it to the desktop.

Is it like we create a DDL file whereby we can keep all the class in here and afterwards we link it to the aspx page?
Thanks for ur help all these while.