Web parts represent a block of content or combination of content and functionality. They are the basic building blocks of portal engine page templates. Using existing web parts, users with the appropriate permissions (Design website from the Modules- > Content permission matrix) can build or modify the structure of pages directly from a browser by using the CMS Desk -> Content -> Edit -> Design interface. All web parts must be placed into web part zones.
From a developer's point of view, a web part is a user control (ASCX) that inherits from the CMSAbstractWebPart class. You can easily create your own web parts as described in the Developer's Guide -> Development -> Web parts -> Developing web parts topic.
The Kentico CMS installation contains many built-in web parts, but sometimes it may be necessary to modify the behaviour, design or functionality of one of them. The Modifying web parts sub-chapter of the same chapter in the Developer's Guide covers several ways how this can be done.
All web parts have the option of using an AJAX UpdatePanel. More information can be found in the AJAX support topic.
Also of interest may be Widgets, which provide functionality similar to web parts, but allow further personalization by users.