Read Understanding .htaccess by Lauren Brumfield (labrumfield.com)

All Reclaim Hosting servers run Apache Web Server Software. So when an account is provisioned the server creates a directive telling Apache what a user’s domain is and where the files for that domain are located on the server. A single server is able to host multiple sites this way because Apache...

Some great basic information and resources here for understanding and working with .htaccess.