Web Part is a modular and reusable component that can be placed into any SharePoint web page (Web Part Pages). A Web Part is generally composed of a Web Part description file (.dwp) and some external code that provides the functionality (a .NET assembly or .dll file).
The Web Part description file is an XML file containing all the property names and settings for the Web Part. The description file also contains a reference to the Web Part assembly. In general, you will never have to look at or understand a DWP file unless you are developing your own custom Web Parts.
In SharePoint, you will find many out of the box webparts which come with the product and they are categorized according its function or purpose.
