Technically, content is every piece of text and media that is distributed/displayed via your site, whether that be in the component, module, mambot/plugin, or template. In Joomla, when someone says content, they are probably referring to the core content component and the content items that are entered into that component and displayed on your site.
The Joomla Language
Joomla's roots are going back more, than a decade (remember, Joomla is a fork of the Mambo CMS, and the development was started in 2000) , - lot of time in the Internet's dog-years, and it definitely has a language of its own. Seasoned veterans are fluent in Joomla 's own language, but for "newbies", this speak can be confusing at best, if not unintelligible or meaningless talk or writing; gibberish in nutshell. This fact can be a main obstacle for ones want to start using/mastering Joomla. This is a reason we began to compile a Joomla-to-English dictionary to help "translate" the usual Joomla-talk. The list of the most commonly used words and/or phrases in Joomla you'll find here is incomplete, and the given definitions are arbitrary. We just attempt to find some "translations" that actually meaning something to a non-veteran Joomla user too. The list is open, the content is not nailed down, any suggestion/addition/request is highly appreciated!
Mambots where the ancestors of current version Joomla plugins. Mambots were introduced in Mambo and offered possibilities to add plug-in logic to your site mainly for the purpose of manipulating content. As the smallest Joomla extensions, they where smaller or bigger chunks of code executed upon being triggered by specially formatted keywords/signal words inserted in the various places of a Joomla 1.0 site.