Everything to do with using your own website to advertise your rental property. Design, usability, hosting, getting listed on the search engines, optimising your site, pay-per-click, etc, etc.
Having your own domain name is much more professional-looking and fully portable if you decide to create your web site via a different channel. Most will also come with email addresses reflecting that domain name which you can use to handle your enquiries and bookings which again is more professional than web mail such as Gmail or Hotmail.