The Administrator back-end of a Joomla website is the area where a user such as a Manager, Administrator or Super Administrator manages all aspects of a Joomla website, including the content, much of the behavior and functionality, and the look and feel of the front-end as well.
In a Joomla powered website, the front-end is the area that visitors interact with, as opposed to the Joomla Administrator that is used by developers and site admins to manage and update the site from the back-end. The site Template is used to display the Modules and other content in the front-end of Joomla.
Metadata has been a very hotly debated issue over the years, and it is now thought of as quite a bit less important in terms of search engine rankings and overall Google performance. Tags such as description are still very useful, but the approach to metadata in web development has changed dramatically.
In a Joomla setup, the server is the system including the web server, which is usually Apache, and a database. These are often on the same machine, but they can also be on different systems in a distributed server environment.
In a Joomla setup the client would be the end user who is requesting web pages and information from the server that Joomla is stored on. However, you can also view the Joomla installation itself as a part of the server.
While iframes aren't used nearly as much as they were in the past, they can still be useful in situations where you are trying to display either an entire website or multiple pages of an outside website while keeping your users on your own site. With an iframe wrapper, even the navigation of a third party website will be displayed within the module or content area.
Banners allow site owners to insert images into one or more module positions, and although this is mostly used for advertising, it can be used for any image.