A modern web page is usually very interactive and you need submit data to server to get what you wanted (e.g. to query on availability of flight ticket). In technique world there are mainly two methods to submit data. One is called GET while the other is called POST.
GET method simply appends your data to URL. It is preferred in most cases because the same data could be shared with others by copying the URL. So a page generated by GET method does be directly accessible by URL. However everything has its downside. GET method should not be the right choice if privacy is an issue. Well, I am definitely sure you don’t want your password to show up at address bar when someone starts typing the site address that you ever visited several days ago, and there should be people who doesn't want others to know the flight that he is going to take, even though you never mind.
Therefore, it is always POST's stage for applications with privacy to be correctly managed. POST method packs data in a chunk that is invisible to user, and not accessible once page closed. Perfect? The downside is you can neither share query result with friends, nor save it in bookmark. So every time you want to recheck the result, you need start from the input page, fill out the form and click submit button. In more complicated cases it may take 4 or more steps (pages) to reach final result.