The world wide web (www) is one part of the internet.
It's the network of web pages, stored on computers around the world, which makes the internet a fun and interesting place to explore.
It's based on a programming language called html (invented by Englishman Tim Berners-Lee) which allows pictures and text to be displayed on computer screens.
It also allow 'hyperlinks', which are the bits you click on to take you to different pages and other related websites.
This is 'surfing the web'.