Understanding a little bit about what Joomla is and how it works will make using it a whole lot easier - we're going to try to explain the basics.
Joomla is an Open Source Content Management System (CMS). Let's break that down into two things - a Content Management System is a piece of software that runs on a server and delivers a website to your browser (Internet Explorer, Chrome, etc) when you request it. It allows the page you see to be dynamic (it can change depending on what you're doing) and it allows the owner of the site to make changes to the way the site looks without changing the 'code' behind the site.
Open source is a concept that basically involves allowing a community to develop and improve a piece of software. None of the people involved in the Joomla project (and there are tens of thousands) are paid, they're all volunteers. The Joomla community do what they do to make the web a better place.
So, in a nutshell, Joomla is a community developed piece of software that allows almost anyone to manage and update their own website.
There are two key parts to a Joomla website, one controls how the site will look and the other the information that you see, the content. How a website looks is controlled by something called HTML, this is the language that your browser 'reads' - from this, your browser knows what colour to make the text, where to put a certain image, how far away one thing should be from another and a whole host of other things.
The content part of the site (the text) is delivered 'dynamically' from a database, which is seperate from the HTML part of the site. The database just stores plain, unformatted text, but allows the web page to connect to it, so when you look at a Joomla website page in a browser, the two are brought together and displayed - the HTML controlling how everything looks and the database providing the content. When you click a link to a new page, the HTML mostly stays the same, but the browser requests new content from the database, depending on which link you clicked.
You might imagine, after reading this, that a Joomla website is a very simple thing, but it isn't! We've tried to write this in a way that almost anyone could understand, but rest assured, a Joomla (or any other CMS) website is a staggeringly complex thing - a typical Joomla site, for instance will have somewhere around 7000 files and a complex database behind it. What we hoped to do, was to give you an idea of what's going on behind the scenes, so that our other guides make a little more sense.