My explanation:
MySQL is like a file cabinet with a bunch of folders with data within those folders, and can be pulled out at anytime (think of a dental office or school). On the web, MySQL is an online database which holds data that can be retrieved whenever requested (for example by a news management system). In fact, most of my PHP projects include MySQL databases to store data needed for the website!

Description:
MySQL is used in web applications and acts as the database component of the LAMP software stack. Its popularity for use with web applications is closely tied to the popularity of PHP, which is often combined with MySQL. Several high-traffic web sites (including Flickr, Facebook, Wikipedia, Google, Nokia, Auctionmarts and YouTube) use MySQL for data storage and logging of user data.