HTML:

When a server answers a Request ,the server usually sends some type of content to Browser ,so that browser can display it .Server often send a browser a set of instructions written in HTML .The HTML tells the browser how to present the content to the user.

HTTP:

Most of the conversations on the web between clients and servers are held using HTTP protocol.

The client sends the HTTP request and the server answers with an HTTP response. HTTP is the protocol clients and servers use on web to communicate.

HTML runs on top of TCP/IP.

TCP is actually responsible of making sure that a file sent from one network node to node ends up as a complete file at the destination, even though the file is split into chunks when it is sent

IP is underlying protocol that routes the chunks from one host to another on their way to the destination.

HTTP then is another network protocol that has web specific features, but it depends on TCP/IP to get complete request and response.

The structure of HTTP conversation is a simple request/response sequence, a browser requests and a server responds.

We can say Apache is an example of a web server that process HTTP requests and Mozilla is an example of a web browser that provides user to view documents returned by web server.