Wordpress is the most popular website content management system (CMS) in the world, by a long way. According to some sources, over 40% of all websites use Wordpress, and you'll almost certainly have visited a WP website in the last few days whether you realised or not.
Wordpress started out as a community-driven blogging platform and it still has this at its core. Over the years, though, developers have built plugins and templates to extend its functionality so it can be used for websites of many different types. There are tens of thousands of themes and plugins that modify Wordpress's functionality, but not all of them will be very good...
There are plenty of agencies out there building websites using Wordpress, and indeed Storm12 has built a few WP sites in the past. Although Wordpress has its place, our experience has told us that it's not a platform we're happy to recommend for most of our clients. Basic WP websites can be very cheap, but they offer limited, inflexible functionality. Sophisticated WP websites add the missing functionality and flexibility, but can cost just as much (if not more) than alternative tailor-made solutions. In both cases the CMS interface can be confusing and hard to learn.
Also, Wordpress websites require constant maintenance and patching to remain secure. Plugins may become incompatible with each other or with Wordpress, and so a required security patch could force a rebuild of a website. Unless you built your Wordpress website yourself and are happy maintaining it you're likely to be paying your agency frequently to update, maintain and fix it.