You should clarify your question, but I'll try and guess at what you're after.
Developers of servlets often name their servlets whatever they want. For instance you could have a servlet named GetWeatherServlet, that an end user could access via getWeather.do or weather.do or whatever you won't to call it. You would define these mappings in web.xml. Example:
You don't have to call it ".do" however, you could call it whatever you want as long as it matches the user's request.
The *.do is usually a mapping for a single Controller Servlet that uses parameters and the values before the ".do" signifier to determine what to do. It is a central concept to many MVC frameworks like JSF and Struts.