"HTTP Redirection" – the process by which a request for one URL is automatically replaced by a request for a different, usually related, URL.
HTTP Redirection is usually accompanied by a Status Code that reflects the reason for the redirect. Most common are 301 (permanent) and 302 (temporary) redirects. If you're not sure which one to take you probably want a 301.
These are colloquially referred to as "error codes", the most well know being
404, but in fact many are not errors.
Redirection can be triggered in several ways
By the Web Server itself
Modern Web Servers such as Apache (see apache) have mechanisms that allow redirects to occur at the server level before any other code (such as php) executes. In Apache, this is done via
.htaccess rules (see htaccess. Other servers have similar mechanisms. WordPress uses this feature of the server, in conjunction with its own URL processing, to construct "pretty permalinks".