The World Wide Web as we know it was developed in 1990 as a means to distribute documents amongst physicists working on a variety of different computer platforms. The Web's creator is a guy named Tim Berners-Lee, and at the time he was working at a high energy physics lab in Switzerland called CERN. Now he works for the W3 Consortium, a standards body for the Web, hosted in North America by MIT.
To emphasize the web's origin, the idea was to be able to format documents in such a way that they would look pretty much the same regardless of what type of computer you used to look at them. There was a server which brokered requests for the documents from clients which displayed the documents after receiving them over the network.
HTTP (HyperText Transport Protocol)is the protocol which describes how documents are requested and transferred between client and server. It specifices things like the parts of a document, the order in which these parts are transferred, the types of environment variable that the server understands, etc. HTML (HyperText Markup Language) is the standard for "marking up" documents with formatting tags which indicate to a client program (browser) how a document should be displayed. HTML is a sub-set of a broader formatting language called SGML (Standard Generalized Markup Language) which postulates how a variety of types of documents could be organized and distributed.
So in the tradition of many fine Internet programs and protocols, the Web is build on the open standards of HTTP and HTML. The weird thing is that people like the web so much, that they want to go beyond the standards and make the web do things for which it was not designed. Meaning, the original intent of HTML was to publish fairly simply formatted academic papers. But as a wider variety of types of publishers began to use the web, they wanted to add various bells and whistles to their documents. Individuals and companies began to add new features to their browsers to allow more sophisticated forms of document layout, and these efforts have generally gone way beyond the letter of the HTML standard at any given moment.