The face of web pages (what the user sees)
HTML is basically the non-moving front-end (what the user sees) of a web page. (Images, forms, text, etc).
CSS is a simple way to define how things look on your page. (For example, making any text on a page show up in Arial font, or changing the position of certain elements on the screen).
runs only in people's web browsers. (It's a client-side language) It is
used to do such things as open up new browser windows, change text or
images dynamically on the screen, validate and then submit forms, and
lots of other dynamic things on the user-end.
The brains of web pages (what the user doesn't see)
PHP is a language that runs on a server.
(It's a server-side language) It can access databases, save and open
files, manage user logins to webpages, and more.
MySQL is (freely-usable) database
software that allows you to store information. (Such as usernames and
passwords, credit card information, etc)