WordPress is a web program, software that runs a blog or website. Originally a nice, simple little blog-script, it encountered success early, and was soon no longer simple, or little. Still, it retains a degree of restrain in it’s design & implementation that establish & maintain a watershed of sorts between it & other Content Management System programs. CMS is the term for software that runs a real website as opposed to a simple blog, which are regarded as scripts (don’t ‘manage’ much).
Like other examples of the fast-moving Internet paradigm, it ‘came out of nowhere’ and within a few years rose rocket-fashion to become a colossus.
You can download the program from www.WordPress.org, and then install it on your own server resources, or you can go to www.WordPress.com, and they will give you an installed copy, hosted on their own servers. (If you know how to do web stuff, do it yourself; if you don’t, let them host the site for you.)
I installed and learned to use several Content Management Systems or CMS (as web-application software is generically known) before WordPress emerged. Each of them utilized in the vicinity of 100 database tables, to store content and to manage the website. WordPress at the time used, iirc, 5 (five) tables.
WordPress now has more tables (11?) than it originally did, but still not many; they remain relatively simple as compared to ‘traditional’ CMS tables, and the interrelations among the tables are fairly intuitive & transparent. Within the traditional CMS paradigm, understanding the Byzantine relations among tables is … something that very few achieve, yet is essential to knowing how the system works.
WordPress is not the only popular-level CMS game in town, but that’s a useful first approximation. And it’s making serious inroads in the high-level game, too.