While technically they both mean “page not found,” Matt Cutts talks about the nuances of each and how Googlebot treats each slightly differently. For those who aren’t too technically savvy, Cutts ...

For example, if a browser requests content that’s no longer hosted on the server, then a 404 (not found) status code will be generated. The first number of the status code indicates what ...

If the server does not wish to make this information available to the client, the status code 404 (Not Found) can be used instead. Google warned once before about misuse of 403s on your site.

The 4xx range of status codes functions largely as expected. Google appropriately processes standard codes like 404 (not found) and 410 (gone), which remain essential for proper crawl management.

If you removed the page and there's no replacement page on your site with similar content, return a 404 (not found) or 410 (gone) response (status) code for the page.

Even non-developers are familiar with the 404 – Not Found status code. Some Web application users may have been exposed to more arcane (and sometimes scary) messages like 500 – Internal Server ...

Google handles them slightly differently. With a 404, and some of the other 4xx status codes, Google will “protect” the page and not mark it as removed for about 24 hours. With a 410 status ...