An email alias is a type of email "pointer" that receives email at a specified address and then forwards it someplace else. One email alias can point to multiple email addresses, and multiple aliases can point to a single address.
One practical use of email aliasing is to create an alternate, easy-to-remember address for a long or awkward email account. This is similar to the practice of using words or phrases in place of telephone numbers. For example, a customer might have trouble remembering a phone number such as 1-800-932-8437 but have little difficulty remembering 1-800-WEATHER. Likewise, if you have an awkward address such as "11243.332@somedomain.com," you may wish to provide an alias such as "bob@yourdomain.com" that is easier to recall and points to the same destination.
You can also use email aliasing to distribute email messages to a particular group of people. For example, if you have a group of friends or business associates to whom you regularly send group mail, you could create an alias called "everyone@yourdomain.com" that forwards mail to everyone in that group.
Finally, aliases can be used to give a small organization a larger, corporate appearance. Instead of telling people to send questions regarding sales, support, employment, and general information all to the same address, you can create aliases at "sales@yourdomain.com," "support@yourdomain.com," "jobs@yourdomain.com," and info@yourdomain.com," all of which could be redirected to a single address.