A URL forwarder is simply a hostname configured to redirect to an actual URL, as opposed to being aliased to some other hostname (CNAME) or set to resolve to an IP address.
An example would be:
example.com → http://www.example.net/something/goes.html
There is no DNS-protocol-specific method to redirect a URL in this way. It's accomplished by standing up a web server somewhere that does nothing but redirect incoming requests, and then pointing your DNS records at it using a host record or a CNAME. The redirect server then needs to maintain a database with the mappings, including the full request URI of the remote destination. It will then need to key on the incoming "Host" header of client browser requests and redirect those requests to the remote destination.
You may hear forwarder or redirect versus stealth or cloaked. The difference...