It is one of the most powerful aspect of sharepoint. Webparts are the main interface elements in sharepoint. Webparts are reusable component that dispalys content on the Webpage in sharepoint.
A Web Part, also called a Web Widget, is an ASP.NET server control which is added to a Web Part Zone on Web Part Pages by users at run time. The controls enable end users to modify the content, appearance, and behavior of Web pages directly from a browser. It can be put into certain places in a web page.