Web parts are page components that provide a combination of content and functionality. They are the basic building blocks of portal engine page templates — you can use web parts to create pages directly in your browser through the Pages -> Edit -> Design interface.
Web parts exist on three different levels:
|Code files||On the most basic level, web parts are user controls (.ascx files) that inherit from an appropriate base class.|
Web parts must be registered as objects in the system. The purpose of the objects is to:
Web parts placed onto page templates are called web part instances. You can create any number of instances of a single web part.
Instances must be inserted into web part zones, which are defined within the layouts of page templates.