I am pretty much interested to know what is the background script that Super User (any Stack Exchange community for that matter) is using to automatically update the content of webpage without reloading the page?
Super User is part of the Stack Exchange network. Stack Exchange is a closed-source platform, and is built on a variety of closed and open source software. The general architecture is described on Meta Stack Exchange and is updated periodically:
Background refresh is powered by websockets, using a custom C# websockets server developed in-house called NetGain, which is now open source. Dynamic interface elements are achieved using jQuery.