What's a CMS?
First things first - let's get on the same page! In brief, a content management system, or "CMS", is software that sits behind a website allowing site administrators to manage pages. They can do a lot more than that, of course, but the crux of their purpose is to enable easy editing of website content. Most good websites have one but there are many different options out there.
Let's talk Concrete5
A bit of background info; the Concrete5 content management system has been around since the early 2000s but has arguably never enjoyed the spotlight or popularity of other systems such as WordPress and Joomla. As with many others it is free to use, open-source and has its own marketplace for plugins and extensions. It's healthier than ever on the development front with new releases coming thick and fast.
Costs, capabilities and customisations
On its own Concrete5 is completely free to download, install and use. You'll need some (PHP-based) hosting of course but there are otherwise no setup costs. An out-of-the-box installation is unlikely to meet your business needs, however, so you might look into themes (some free, some paid) and add-ons (also free and paid options) on the marketplace. This is the same with most CMS.
If you've got a dream and a good developer, Concrete5 can pretty much manage anything. It's capable of being the backbone for simple 'business card' websites, blogs, eCommerce shops, and even more advanced web applications. Whether it is the best option for all of the above is another matter, though.
The core building mechanic of Concrete5 is a drag-and-drop system that works with things called 'blocks', similar to what WordPress now offers with it's Gutenberg update but more visual. You can also create layouts and control certain aesthetic properties of page content from within the page itself. There are plenty of system settings that allow for further tweaking of SEO, performance and general site information. Many things come as is, including social share icons, video players and image sliders.