What is 404 error?

hello,
can any tell me about 404 error.It happens when we are trying to put wrong url in url box.

If someone clicks a link to a page on your site that doesn’t exist, something has to happen. By default, the user will land on a rather plain and ugly page indicating that the page does not exist. The number 404 is the code number that signifies a missing page. Ideally, you should set up your own custom 404 page to override this unhelpful default page. Typically, a good 404 page would apologize for the bad link and offer options to users to help them find what they are looking for—such as a site menu.

Take a look here