Sitefinity works in the same way as each ASP.NET website. We follow the standard ASP.NET structure for a website. When the development is done you can migrate your website to the production.
1. User controls - these controls are not compiled they resides in a folder of the web. application and their code can be changed by a developer easily.
2. Custom controls - you can compile them the dlls and upload to Sitefinity using Controls toolbox section in page edit mode. Another option is registering the controls in your web.config file.
the Telerik team