Hello Rajkumar!
This article explains a lot when it comes to how dynamo works with the revit api: https://github.com/DynamoDS/Dynamo/wiki/Python-0.6.3-to-0.7.x-Migration#revitapi
The Revit API uses .NET assemblies to give access to functionalities in the program, these assemblies are .dll files. RevitAPI.dll and RevitAPIUI.dll are files that can be found in the Revit program folder.
To use .NET with python Dynamo uses a variant called IronPython. How clr and clr.AddReference work is explained in the IronPyton documentation: http://ironpython.net/documentation/dotnet/
Finally when you do from Autodesk.Revit.DB import * you import all the classes in the Autodesk.Revit.DB Namespace, and thats where most of the Classes (Methods etc.) you can use in Revit are. Se all classes in DB here: http://revitapisearch.com/html/87546ba7-461b-c646-cbb1-2cb8f5bff8b2.htm