The wonderful thing about Joomla is that it has an extensive array of add-on's including components, modules, plug-ins and templates. OK, I can see it now. You're eyes are about to glaze over with all this talk about componenets, modules, etc.....
Don't be. Just think of it like a Ford F150 truck you buy and then decide you want front runner fog lights added. So you go to a store, buy the fog lights and install them underneat the front grill. It's an add-on.
With the websites we develop, let's say you need a calendar application and the ability to embed a video on a page. We will install a calendar application and a plug-in to display video into the site. Viola! An add-on.
What kind of add-on's can be put on a website? Over 3,000. There is a full list of the current categories below. Basically, most clients need add-on's for calendar/event scheduling, embedded photo galleries, embedded video's and/or music, on-line documents, contact management as well as ease of adding/editing/deleting content. For those who need e-Commerce, they might need an online shopping cart or for those that just need simple buy-now buttons, they might need a PayPal interface plug-in. Most add-on's are non-commercial meaning they are available for public use but there are some fine commercial addons for those that need that extra bit of functionality.