Recs.
Updated
WordPress is a free and open-source blogging tool and a content management system (CMS) based on PHP and MySQL. WordPress was used by more than 23.3% of the top 10 million websites as of January 2015. WordPress is the most popular blogging system in use on the Web, at more than 60 million websites.
SpecsUpdate
Pros
Pro Lots of plugins that extend functionality are available for Wordpress.org
There are plenty of plugins available to extend the functionalities of WordPress.org, as well as shortcodes in Wordpress.com
Pro Optional free hosting
WordPress offers free hosting under a wordpress.com subdomain. This option eliminates the need for setting up the CMS yourself and is reasonably secure, as WP uses multiple servers to back your site up. Additionally, for a fee, you can even set up your own custom domain name.
Cons
Con Clunky, and trying to do too much
If you stick to basic blogging, you might be ok. But, if you get too excited and picky and want to customize too much the appearance & features, then you'll fall into an abyss of clunkyness (code, 1000s of plugins, documentation, support, hack-o-ramas). This might be ok for some, or not. Consider how many hair you have left beforehand ;-)
Con Spam
Wordpress by default allows commenting on pages, any page. Even if you turn commenting off for a page bots can still access the commenting endpoint; even if you use a 3rd party commenting system such as Disqus. Wordpress comes with an anti-spam filter called Askimet which does a pretty good job but not perfect.
Con Self-Hosted
Wordpress.org requires you to have a domain name and web hosting provider to get started.
Though it's important to note that Wordpress.com offers free managed sites with hundreds of themes, and numerous other managed wordpress providers (wpengine, godaddy) will set up sites for you.
Con Wordpress.org allows too many amateurish, insecure, outdated, unmaintained, incomplete or useless plugins
Wordpress.org allows plugins. For security reasons, Wordpress.com does not. Other managed Wordpress sites allow only a smaller "curated" set of plugins.
True, there may be some plugin for every functionality you require. However if you take a more detailed look at those plugins they are either outdated, made by some hobby programmer (i.e. no in-depth testing, no security audits, no code reviews, hacky, unmaintainable) in their spare time (and don't get me wrong: I adore everyone giving something to the community; but many of these plugins are just unusable for serious business), incomplete (regarding multi-language capability, an author of a famous Wordpress form builder plugin responded something like: "Well, maybe sometime"; seriously, man?), insecure (e.g. recently there was a serious flaw as a buggy plugin is used by many themes) or often need much hacking to finally get the correct functionality that YOU need currently.
Recommendations
Comments
Flagged Pros + Cons
Con Data based content requires plugin
If you want to force your users to enter information in a field based way you need an extension such as Advanced Custom Fields.