Hypertext Markup Language is a language used to represent text on web browsers, combined with CSS (Cascading Style Sheets) it is used to create most websites on the internet. There are no standards with this (look up ISO standards) so the quality of websites vary A LOT depending on the programmers ability. This is why there are libraries/tools/frameworks such as Bootstrap (http://getbootstrap.com/) to aid new web develops in creating good looking sites, the problem with this is that they all look the same! no personality!
HTML describes the layout/contents and CSS describes how to render things on the page.
Links:
- http://www.w3schools.com/html/default.asp (Many more tutorials, see the navigation bar)