What are 404s?

404s are the technical term for "missing content" on your site. Basically, there's a wrong address somewhere, or someone's trying to access information you have deleted or never published.

They are very low-priority items, but fixing them improves your site usability, and can have a positive effect on search ranking. Many SEOs believe they have very little/no influence: 

Large numbers of 301 redirects and expired/low value URLs that 404 won’t directly cause any general ranking problems for the site (assuming you’re doing them properly).

https://www.deepcrawl.com/blog/best-practice/expired-content-for-seos/

How do I find them?

Glad you asked! How to find broken links.

What should I do with them?

• If you move content to a new location, we should do a 301 redirect.
• If you delete content, we should remove any links to that content then de-index it.

What?

Let's say Sally is on your website looking at your About page. She sees a link to Tom's profile and clicks it, visiting Tom's profile and making contact with him about same sales opportunities. Nice work! But Tom leaves the company! The next time Sally goes to find his profile, it's gone and she gets a 404 (not found). What's the best experience?

1. We should definitely remove the link to Tom's profile from the about page. That's a no brainer!
2. The page is still in search engines though, so we want to de-index it. We can put in a request on Google Webmaster Tools/Search Console to take it out of the largest search engines.
3. Now, no-one should find it, but if they do, they'll get a a 404 page: content not found. That's the right thing to do. A 'smart' 404 page might have links to important pages and a search function to help the user out.

The content was removed and there's no new address: remove links and de-index.

As the company grows, it's crazy to have all the staff on the about page, so we add a /staff page. At some point, we decide that the choice of language isn't great, and we want to talk about People, not Staff. The /staff page is moved to /people ... and Houston, we have a problem. There are links to /staff all over the place and it's a popular page. It's linked in industry forums and all sorts! What's the best approach?

1. We change all the links on our site from /staff to /people -- that helps everyone on our site find the new address.
2. We do a 301 (moved permanently) redirect to help people from outside links find the new address. This also means search engines pass on any "link juice" to the new address so that you don't lose search rankings.
3. So everyone (and every bot) arrives at the new address without issues.

The content was moved, and we helped everyone find it: update links and 301 redirect.

Note on social: It is impossible to do this trick with social share numbers. Those are locked to a unique URL and cannot be passed to a new address. Each address starts at zero social shares.  

Note on search rankings: And here's a note on 3xx redirects and search engine rankings (late 2016) -- they theoretically don't impact results, but there are inherent risks.

What do I actually need to do?

You can send us a spreadsheet of broken links and new addresses in two columns (old address | new address), or you can buy a complete re-direct report from Performance Foundry to take care of the research and the work for you. Here is a CSV file we created as a  template.

If I get other errors?

Take look at this write up on  server errors and how do deal with them.