A user level on a Joomla site that by default can access only the front end. Editors have privileges to post and edit content, but no permission for publishing content.

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 #__ to represent the prefix. This will be replaced by the real prefix during runtime by Joomla.

Reference: http://docs.joomla.org/Glossary

The main Joomla back-end Control Panel which is used by administrators for managing the whole site.

Module positions are placeholders in a template. They identify positions within the template and tell the Joomla! application where to place output from modules assigned to a particular position. The template designer has complete control over module positions, creating variations between templates and the respective Joomla! default positions assigned to modules in the installation sample data.

For example, the module position Left could be defined to be on the left side of the template to display a site navigation menu. So if a module is assigned the Left position, it will be displayed wherever the designer puts that Left module position - not necessarily the left side of the page.

Reference: http://docs.joomla.org/Glossary