Here at DreamHost you can host 'unlimited' domains or sub-domains under any of their hosting plans. Each domain or sub-domain will have its own separate web directory that you can upload your site files to, so they are basically independent.
The discount offered by the user created promo-codes comes out of the $97 rewards bonus that DreamHost pays the creator of the promo-code for referring a new customer.
The exact discount offered is up to the creator of the code, some choose to offer the full $97, while others (like myself) prefer to offer a $50 discount (or some other amount).
Those that offer $97 receive nothing from the referral, unless the new customer chooses a monthly plan, where the max discount allowed is somewhat less than the full $97. They can also receive referral rewards for secondary referrals.
Edit: I missed your last question.
Yes, Wordpress is offered as a 'one-click install' via the admin panel and the process really is simple. You simply fill in a few text fields related to where you would like the application installed and database creation etc, click a button then wait for the email telling you it's all been done
Save [color=#CC0000]$50[/color] on DreamHost plans using [color=#CC0000]PRICESLASH[/color] promo code (Click for DreamHost promo code details)