Overview
You can preview how your site will look and work on DreamHost before pointing your domain's DNS there, using either a 'hosts' file edit on your computer or a free dreamhosters.com subdomain. Your site keeps loading normally from your old hosting company until you complete that DNS change.
Which option should I choose?
How do I view my site using a 'hosts' file?
This option is the most accurate way to view your site as it appears on DreamHost's servers, however it's more complicated as it involves editing a configuration file on your home computer. If you are uncomfortable with updating configuration files on your computer, use the dreamhosters.com option instead.
How do I view my site using a free dreamhosters.com subdomain?
A dreamhosters.com subdomain is a free subdomain that allows you to see what the site will look like at DreamHost before you make the final DNS change at your old hosting company. This is the easier of the two options as it does not involve editing any configuration files. You can create and manage the sites within your DreamHost panel.