A domain-alias allows multiple domain names on the same hosting account. This also means that anything sent to your email@domainname1.com will also be received by email@domainname2.com, which means you can manage all of your e-mail in one interface for both domain names.
This also means that domainname1.com will look and will pretty much be the same as domainname2.com. It's different to domain-forwarding as this is a service that forwards all traffic from domainname1.com to domainname2.com as its main site.