I’ve found the best approach when learning about something new it to keep it simple (stupid).
I found this basic and incomplete diagram (resplendent with typo) to help us out.
HTML – Stands for Hyper Text Markup Language and it is the building-blocks of web pages. It’s a programming language that allows you to add text, images, links and other pieces of content to your sites. It’s there to create the basic structure for a website.
CSS – Stands for Cascading Style Sheets. This programming language creates the look and feel of the website. Font size, colour and type, as well as styling around images, page layout, mouse-over effects and more are determined by CSS.
PHP – means Personal Home Page. It’s also a programming language that makes a web page do things, but it doesn’t run in your browser. It runs on a webserver. PHP is generally used for giving and receiving data to and from the database.
For the Visual Learners
Another simple diagram looks like this: