What are WebParts and how do they interact with SharePoint?

WebParts help build reusable components which can be customized and personalized according to a business user. We can either make our own webpart or reuse existing ones from SharePoint.
The following WebParts are available with WSS:

• Data View Web Part: Displays data with rich design support through Microsoft SharePoint Designer.
• List View Web Part: Helps us display list content for any list in the SharePoint site.
• Image Web Part: Helps us display image files.
• Content Editor Web Part: Use this to display static HTML content using a WYSIWYG editor or to link to a text file.
• Members Web Part: Helps us display members of the site.
• Page Viewer Web Part: Displays a web page in an iFrame.

WebPart is nothing but a simple class which inherits from System.Web.UI.WebControls.WebParts. In other words, you can say WebPart is kind of a WebControl which can be deployed in a WebPartZoneControl.

WebPart manager control is like a controller which maintains instances of WebParts. It adds WebParts to the WebPartZone when ASP.NET pages get initialized.