The core Joomla software is made even more flexible by the use of third party extensions created by Joomla Developers to customize your Joomla site.

However, you're not going to be able to build anything, unless you understand the terminology and concepts on what makes  Joomla run. 

While the Joomla Content Management System (CMS) is great software to build a website, right out of the box, with the proper third party add-ons, you can run a small blog or a mega-store using Joomla.

Third party extensions are divided up into four categories:

Components – separate programs that integrate with Joomla, such as extended statistics, a sitemap, gallery manager or a reciprocal link program.
Modules – programs that usually represent the front end of a core or third party Joomla component, and run outside your content area, such as “Menus,” “Most Popular” or a “Members sign-up/login form.”
Mambots or Plugins – tools to edit or add into your content area or articles
Add-ons – yes, there are third party tools for your third party tools. Sometimes a tool is so popular and useful, developers will write a specific upgrade for it.
Content Area - Where your articles are. Not necessarily the column that they are in, but literally, where the article begins and ends.

The thing you have to remember is that while the most successful Joomla site is highly customized and highly optimized, content is king.

In order to add content to Joomla, you must first create a:

Section – Top Level
Category – Second Level within the Section
and then a
Content Item – Your actual article

You can choose to never display your Section or Content name within your website structure, but they have to be there. In Joomla 1.5 you have an alternative – Uncategorized. So you never have to create a Section or Category for your article. But in 1.0X you have to create a Section and Category before you begin to post.

Since you can add Sections and/or Categories to your menu, I suggest you name them carefully and think about how to best utilize them. You can also display Content Items in a menu.

These are the basic Joomla concepts that gave me fits, when I first started.

I also want to note that third party software Joomla Developers are very helpful. If for any reason your add-on does not work, most developers have blogs with tips and forums with users, helping users.