Email Forwarders allow you to continue using an existing email account (e.g. an account provided by your ISP to recieve email sent to your domain name.
Any messages sent to a specified email address at your domain e.g. yourname@yourdomain.com can be automatically redirected to an existing email address of your choice, just like traditional postal forwarding.
An email Forwarder cannnot be set up with the same address as and existing POP3 Mailbox and vice versa.
The most popular use of an email Forwarder is a temporary one e.g. people who are traveling may want their email redirected to an address the can access whilst they are away from their normal computer.