Web (short for World Wide Web) refers to the mechanism allowing the sharing of information over the Internet (a network of computer networks and computers).
The Web involves the use of Web Browsers (such as Internet Explorer, Firefox and others) to access Web documents called Web pages that are saved (Hosted) on the computer/s of a hosting Service Provider who's computer is connected to the Internet all the time.
A Web Site is a group or set of interconnected web pages, usually stored on the same computer, with a central theme and belonging to a single person, group or organization.
The method of finding or identifying individual web documents is by referring to the identifyer of that document or web page. These identifyers are called Hyperlinks (normally just a word or a phrase or an image) which are placed inside the content of the Web pages. Hyperlinks are used to locate defferent Web pages inside Web sites and become unique on the Internet when they are associated with a Domain name.
Domain names are used to identify Web Sites on the Internet and by nature of it's use need to be unique. There are no two Web Sites on the Internet with exactly the same Domain name.
The combination of a document identifyer and the Domain name is called the Unique Resource Locator (URL) which is the address entered in the address field of a Web Browser (eg. Internet Explorer, Firefox and others)that will recall the stored Web Page identified by the URL.