The World Wide Web (www) is a public hypertext system working in Internet. The web provides access, with a browser, pages accessible in sites. The image of the web comes of the hyperlinks linking web pages between them.
The Web is just an Internet application. The Web was invented by Tim Berners-Lee and Robert Cailliau years before Internet, but it is what made the media to Internet. Since then, the Web is frequently mingled with Internet; in particular, the word Web is often used in the no-technics texts without designated clearly the Web or Internet.