A user level on a Joomla site that has access to all front-end and back-end capabilities. Administrators is just below super user level.

The template used to specify the layout of the administrator back end of a Joomla site. The Back-end Template controls the way your website's administrative tasks are presented for controlling management functions by a Joomla! Administrator. These would include common tasks such as: user, menu, article, category, module, component, plugin and template management.

The name for a specific piece of language-specific text in a Joomla application.

KEY = Value: where “KEY” is a string to be translated and “Value” is the translated string. In a Joomla program, language keys are used instead of literal language strings. The actual text is substituted at runtime based on the currently active language.

Aliases are short pieces of text that represent the title of certain items (Menu items, Articles and Categories) in a machine-friendly format. This format allows only lowercase letters and dashes (-).

Aliases are used by Joomla to make Search Engine Friendly URLs. There are technical limitations to the types of characters that can be included in URLs, so Joomla prevents problems with invalid characters by allowing you to specify an alias.

You can fill in an alias yourself. If you leave the alias field empty, Joomla will automatically create an alias from the Title field of an item when it is saved. This means that if you edit the title of an item, but you leave the old alias in its field, the alias (and the URL that is created from it) will not change. Empty the alias field if you want generate a new alias.

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