Joomla Multisites

Allows multiple joomla-powered sites to be run from the same codebase, as well as wholly, partially or not at all through shared database-tables (If shared, all subsites share one or more individually chosen tables with the main site).

Each website can share or have unique modules and components installed on it.

The different sites can be based on directory, eg:
http://www.yoursitename.com/ (main site)
http://www.yoursitename.com/subsite1 (subsite1)
http://www.yoursitename.com/subsite2 (subsite2)

Or by dns eg:
http://www.yoursitename1.com/ (main site)
http://subdomain.yoursitename1.com/ (subsite1)
http://www.yoursitename2.com/ (subsite2)