The redirect manager is a wonderful idea. It redirects a visitor to a valid page when he enters a path on your website that doesn't exist. It is possible that it existed before and was added to the search index of a search engine, or someone stored it at another website. This problem usually occurs after relaunching a website.
The redirect component is very convenient. If someone tries to access a path that doesn't exist, Joomla! will show the 404 - not found page and simultaneously create a new entry in the redirect manager.
When accessing Components - Redirects, you'll see these links (Figure 1). The System - redirect plug-in has to be activated in Extensions → Plug-ins.
Figure 1: Redirect Manager
You can now edit the links and add a new redirect URL. The next time the old path is accessed, the redirect component ensures that the visitor is redirected to the new address. You may create redirects on your own if you are aware of any old paths that no longer exist.
The component uses the web server's rewrite system. Currently, only Apache is supported with an installed mod_rewrite. In Site → System Information you can see which web server you are using. (Figure 2)
Figure 2: System Information
When using Apache, go to Global Configuration → Site → SEO Settings → Use Apache mod_rewrite and switch it to Yes. Afterwards, rename the file htaccess.txt to .htaccess. In some cases renaming the file may not be possible because the file starts with a dot. I use my ftp client or the shell for renaming the file (Figure 3).
Figure 3: .htaccess