Not sure what exactly you are looking for, your question is nothing that Google can't answer. for e.g. __doPostBack
] this link talks exactly about what you are asking and in detail.
I would recommend Googling questions like "what is XYZ?" first before posting here. Google does a better job at returning links that talk about "what something is and how to use it". These forums are for very specific coding problems that you face that may not otherwise be solved by Googling about them and generally investigating them.