What's a 404?

A 404 error is an HTTP status code that means the page a user is trying to access is not available on a website. This error message is sent from the webserver of an online presence to the web browser that sent the HTTP request. The 404 error code is one of the most frequent errors a web user encounters. It can occur when a user attempts to access a webpage that does not exist, has been moved, or has a dead or broken link. Some websites customize their 404 pages with humor or creative designs to entertain or assist their visitors. On the website owner's side, 404 errors should always be fixed when possible, as having a large amount of broken, unredirected, or nonexistent links builds up over time and creates a poor user experience.
What are some common causes of a 404 error?
How can I troubleshoot a 404 error?
Are there any tools or services that can help me find a specific page that returned a 404 error?