Relational databases are often used by businesses to keep track of employees, inventory, and customers. They are helpful for distinguishing relationships between entities in a company.
The pioneer of relational databases, Edgar Codd, designed what is jokingly known as "Codd's Twelve Commandments". (Technically there's 13 because the number scheme starts at zero). If you want to read about them, the link is here!
Content management systems are how normal human beings (not CS people) manage websites
They are pretty powerful though! Check out the Collegian website, it runs on WordPress. There are a lot of little tweaks and addons you can implement to make a CMS do some pretty heavy lifting.
It's open source. But like all open source code, you should consider paying for the pro version when you're running a business
Creating users with privileges gives you a system of access control
Watch out for DROP DATABASE -- you're gonna have a bad time if you do this on accident. Or someone else decides to do it for you.