HTML (or Hyper Text Markup Language) is a kind of code and can be thought of as a language, with which to write and edit the bare basics of websites. It is basically an Internet standard that you can write pretty much anything into, and it becomes visible or audible.
It is made up of 3 essential parts: Document type specification (<!DOCTYPE html>): This determines that the document in question is in fact an HTML file, and will codify whatever information you write later on as an HTML file.
HTML header (<head></head>): This part contains technical documentation, and is invisible to the end user.
HTML body (<body></body>): This part contains the content that will be visible to the user in his/her browser. These can be active or passive, and can be any kind of file. It also dteremines the format of the information being used, whether it is a picture, music or text file.
The browsers receive the HTML files from a server or a hard drive, and these are turned into multimedia websites, that are then converted into visible, audible websites or webpages for the end user.
The way an HTML website looks is determined by CSS (Cascading Sytle Sheets). The CSS files can be inserted directly into the HTML code, or can be attached as an outside source. CSS determines all stylistic features, from letter fonts to colours used on the site, to the width and height of a given unit.
Most webpages are written in HTML, and it is thus an irrevocably important part of online marketing. We strongly recommend getting at least a basic knowledge of this format, as this will allow you to understand how any website, including your website, works.