Just another set of terms for deranged mind from deranged mind. It's nothing but
1. one drupal instance hosting multiple site and
2. multiple site hosted on different instance of drupal
Some call instance as code base ... it's OK if the terms don't confuse you.
various materials are there on the topic with really easy set of instruction(s) to carry out the task.
For hosting on the public domain  with the multisite/multiple site stuff if one needs to setup an internal network(test site/sandbox) which completely emulates the network setup i.e the domain name, subdomain, webserver, BIND, WHM to test the above jargon then kudos go ahead or else the env can be emulated on the localhost either with the virtual host or without it.
I made use of both the env i.e on localhost as well as setting the whole network(with the help of ehcp) to get multi as well as multiple site conf.

After setting up the env there where some tricks involved using .htaccess and the voodoo in the hosting env - mod_rewrite