doGet method maps to the code executed when your servlet receives a "GET" HTTP request. doPost method maps to the code executed when your servlet receives a "POST" HTTP request. These methods are internally called by the `execute` method of your servlet (a lifecycle method) based on the HTTP method of the request received.
If you have specifically created a servlet which handles only POST requests (uploading etc.), you'd normally override the doPost method. Any other HTTP method used to access your servlet would result in a "method not supported" exception (well technically it isn't *any* other method but you can ignore the difference for the time being).