What is a postback?

POSTBACK: Part of ASP.NET's contrived technique for hiding the true stateless nature of the web/HTTP behind a stateful facade. This results in complex code (IsPostback, ...), a hard to understand page lifecycle, many different events, ... and numerous problems (ViewState size, web-farm stickyness, state servers, browser warnings (not using PRG pattern), ...)

See ASP.NET MVC instead.

So far I've seen the right answer alluded to repeatedly, and almost everyone has come shy of what I consider subjectively to be the mark.

Let's start with the basics:

An HTTP request can be any of the HTTP verbs, but the two that people use most are GET and POST. Well, those are the two a programmer uses most frequently. The others all have some purpose, if they're implemented on the server. When you send information to the server, you can do so either through the use of the URL (to request a page) or within the body of the request (POST, PUT, DELETE, for instance).

Now you'll remark (I'm sure) that the URL in a GET request often contains data, and this is true, but according to W3C, you should not use GET to alter state, and yet we do often. It's sort of a hack that we all agree is an actual use, and not a hack. Whether that makes it a hack or an actual implementation detail I leave up to you.

So when you send the body of the POST (skipping the others for now, you can figure it out from here) with the form elements, you're sending back certain elements. How those elements are defined is up to you and to the environment you're working in. You could post to a server with a JSON element in the body, or with XML, or with form fields. Generally we do posts from a FORM element in the body of the HTML.

Now everyone says, "oh, a postback is a subsequent request to a page." But, that's not true. A postback is when you send data via POST -> back to the server. I say this because the difference between a GET request and a POST request is if data is included in the body (and the verb used, but the client usually knows how to deal with that). You could postback to the page on the first time the page is visited, and in fact ASP.NET has tools for doing that in the library. You could certainly have a desktop client POST data to a server (think Twitter) without showing any webpage at all from the server (ok, so twitter is probably not the best concept to use for an example here, but I want to illustrate that you can use a client that doesn't show the webpage, so no request is necessary).

So really what you should read there in "postback" is "I'm POSTing data BACK to the server for processing". It's presumed that you retrieved the page initially with a GET to show the user the <form> element that has <input> fields for them to interact with, and that at the end you're sending data back. But I hope you can see that it doesn't have to be in that order.

So here's something else to consider:

What if you gave the user a page with a bunch of <input>s and no <form> but instead, had a button wired up in javascript to concat all those <input>s with &value-n= and send them as a GET? Does the same thing, but violates that concept of only using GET for requests. (possibly) ensuing discussion encourages me to reinforce that GET should have no side effects (no updating values)

It's how come you can send someone a link to a google search, for instance. So we don't ALWAYS have to POST BACK to the server to get data.

Hope this helps. Cheers

See ASP.NET Page Life Cycle Overview on MSDN for a good general introduction about what happens when a requests hits the server.

A PostBack is any request for a page that is not the first request. A PostBack will always be in response to a user action (triggered most commonly by a Button, AutoPostBack control or Ajax).