Database prefix The database table prefix is a string (a few characters long) prepended to the name of Joomla!'s tables. Using a prefix enables you to run multiple installations of Joomla! using a single database. The database table prefix can be set during installation. Changing it later is possible, but requires access to the database through a non-Joomla medium or a Joomla Extension such as Akeeba Admin Tools and will cause some downtime. Extension developers need to use the string j73172_ to represent the prefix. This will be replaced by the real prefix during runtime by Joomla. Reference: http://docs.joomla.org/Glossary
XHTML It stands for Extensible hypertext markup language and is an application of XML. Most Joomla pages are based on XHTML.
JED (Joomla Extensions Directory) The official directory for all components, plugins and modules available for use with the Joomla platform. This database is maintained by the Joomla developers.