Multi-Site, Multi-Domain, Multi-lingual Web Sites

I was wondering what and how these multi-sites, multi-domain and multi-langauge sites actually differ.

First of all I will talk about multiple sites and multi-sites.

In Multiple Sites, apache control the one to many relationships between a single IP and many "websites" using virtual hosts.

For Multisite deployment Drupal approach is best where the same codebase and database to host multiple instances of drupal is used.

I have read the write-up:

http://lno.lternet.edu/node/192
http://drupal.org/node/1105042

Do the development of these sites have to do with creating a sub-directories, sub-folders or sub-domain and how does these approach effect SEO??

Could anyone please share views or personal experience on this, your comments are awaited. Keep the ball rolling!