Yes, of-course, hosting multiple domains under the one account is one of the great features of DreamHost.
If the domains are registered here at DreamHost, all you need to do is add them to the hosting system via the admin panel at Domains -> Manage Domains -> Add New Domain / Sub-Domain, you add them as Fully Hosted.
If the domains are registered elsewhere, there is an extra step you will need to do before doing the above. You need to configure the domains DNS to point to the DreamHost DNS servers. You will do this using whatever configuration utility your domain registrar provides.
The DreamHost DNS servers are...
Note, after changing your domains DNS configuration, it will take some time before the changes propagate around the Internet.
After you have completed the above steps, your home directory will contain a sub directory for each of your hosted domains. These will be named the same as the domain name (ie: mydomain.com). These sub-directories are where you place the site files for each domain.
edit: Beaten by matttail, I must learn to type faster
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