PostBack in ASP.NET

Lets consider a .aspx page in ASP.NET which consists of a some fields having a Submit button at its end.When we click on submit button,the page is submitted back to server for processing along with data on page.Finally our data gets saved in the database.This is due to the PostBack property.This property runs our code on server side to fulfill the task.Similarly suppose if we have dropdownlist which are bound to database can be viewed on Page Load.But there is no need to fill the dropdown once it is filled.For that ASP.NET provides us a functionality to check whether the Page.IsPostBack property is true or false.It is true for the first time page on load but in all other operations on the same page it is false.So it will not fill th dropdown again & again and save our time as it will not connect to the database again.
This property was not present in Classic ASP.Hence the code was executed line by line from top to bottom.To avoid this we use iframes in that which was more tedious job.
This is what Postback in ASP.NET means