Email forwarding involves passing email along from one address to another. It can also be used to pass groups of messages such as listservs.
Email forwarding services automatically forward all mail received at one of your addresses to another address. This can be handy if you have a web based email alias but would rather not have to surf to the website each day to check your mail. Using a forwarding service, the mail received at the website would automatically be passed to your main email address (or another address of your choice). This allows you the convenience of utilizing multiple email addresses while keeping your main email address private, and still collecting all of your mail in one place.
Mail forwarding services are also handy for people that change their ISP (and therefore main email address) often. With some forwarding services, you are simply give out our one address no matter what ISP you have; the address might look something like: yourname@forwarding-service-provider.com. Friends will always have a current address to send mail, and when your main address changes you only have to notify one party (the forwarding service) rather than dozens.
Many services also offer SMS alerts for mobile email notification and other advanced management features. For example, the email that gets forwarded to you might automatically be distributed into a variety of accounts using filtering technology. Thus, while you can give out a single address to all, the incoming mail would get sorted into categories as it enters your email client. This can be done by setting up several identities in the client. A work identity, a family identity, one for friends, one for listservs or mailing list, for example. With one click you would collect your mail from the forwarding-services's site using their POP3/IMAP server in your configuration. As the mail is retrieved it is automatically sorted into the proper identities for easy management.
In the same way, mail can be sent from your email client using the SMTP server that belongs to the mail forwarding service. This allows you to use an email client rather than the website where the mail forwarding service is located, though you can always use your browser and the website interface instead.
Though using an email forwarding service can be handy, bear in mind that web based email is generally not considered as private or secure as mail kept on an ISP mail server. Web servers are more exposed and the expectation of privacy is usually lower. You may not want to use a service like this for very personal or sensitive information. In any case privacy policies and Terms of Service agreements should be thoroughly read and understood before signing on with any company.
There are several email forwarding services available, easily found with a search engine. Free services usually require the client accept a certain amount of spam or advertising from sponsors. For a modest fee this requirement is usually waived.