During the life of a website, it is common for the page structure to change. Sometimes a page gets moved to a new location; sometimes it gets removed altogether. In any case, it's worth considering how to respond to traffic to pages that no longer exist where they once did. Is it suitable to redirect users somewhere else? Or is allowing them to experience a "broken" link more appropriate?
There are two main ways to deal with traffic to a page that is no longer there.
1. Do nothing: Allow Page Not Found (404)
By default, if the server cannot find the requested page, it will show the user a "Page Not Found" page. In doing this, the server also sends along the code "404," signifying to the browser that it could not find the page. This is why the "Page Not Found" page is sometimes called a "404" page.
2. Designate a new destination: Create a Permanent Redirect (301)
Instead of saying it can't find the page, the server tells the browser that the page has moved. It sends along the code "301" (signifying the type of redirect), as well as the path where the browser should go. When this happens, the user will not see a 404 page but will go directly to the designated page.
The question to ask is: Does the content of the missing page, or its equivalent, now exist somewhere else?
When the Content Still Exists
If the content does exist, best practice is to redirect any traffic from the old location to the new location.
A redirect ensures users end up with the content they were expecting. It also signals to search engines that they should update the URLs in their results.
On redirects, Google has the following advice:
If you need to change the URL of a page as it is shown in search engine results, we recommend that you use a server-side 301 redirect. This is the best way to ensure that users and search engines are directed to the correct page.
If you set up a redirect, it also has the benefit of keeping alive the SEO value of the previous link. When links are permanently redirected, search engines tend to transfer much of the reputation of the original link to the new one.
When the Content No Longer Exists
If the content no longer exists, then letting the link go to a 404 is more appropriate. Allowing users (and search engines) to hit a 404 isn't all bad. 404s in themselves won't destroy the SEO value of the site. In fact, search engines prefer 404s so they know which links to remove from their results.
According to Google’s Webmaster Central:
404s are a perfectly normal part of the web; the Internet is always changing, new content is born, old content dies, and when it dies it (ideally) returns a 404 HTTP response code. Search engines are aware of this... In fact, we actually prefer that.
In general, if the content of a former page still exists somewhere else, set up a redirect to the new location. If the content is gone, allowing users to see a 404 page is a valid, and often preferred, solution.