HTML which stands for HyperText Markup Language is the form of marked-up text used on web sites.
In principle it would be possible simply to write words, in much the same way as most word processors can save documents as plain text. The snag with saving a word processed document as plain text is that all images and formatting are lost. HTML provides a way of including formatting information within text.
Its origins were simple drawing on the ideas of the more general Standard Generalised Markup Language it allows instructions enclosed in angle brackets to be mixed in with text and tell a browser (such as Firefox, Internet Explorer or Safari) how to lay out the page. An example of how this works is that putting <i> tells a browser to switch to italics, and this is cancelled by </i>. Putting this in my HTML:
this <i> has some words in italics</i> and some not
this has some words in italics and some not
Web browsers are supposed to read the HTML and display it according to a standard specification: one of the complexities of web design is that they are not as consistent as they might be.
A second complexity is that the the designer has no control over the size of the window in which a web page is viewed. It is possible to resize the window in which a web site is viewed, and a full size window to someone with a wide monitor could easily be double the width of full size to someone with a smaller monitor: both people are likely to see their monitor as normal. As an illustration of that, try re-sizing the window in which you are reading this page. The silverfen-web.com web site has been designed with a minimalist look, but care has still been needed to make it work at different sizes.
The idea behind HTML is that it should be capable of being written with a good text-editing program. Since it was first invented the range of HTML tags has gone up, and while writing by hand probably remains the best way, many people today use special HTML-editing programs (such as Seamonkey, CoffeeCup, Amaya or Dreamweaver) to avoid the need to learn all the tags. While this can make life easier, it also tends to leave people not really understanding what is happening in their HTML, which can lead to problems.
These days HTML can be both simpler and more powerful than when first invented, because the use of style sheets make it possible to (re)define effect of HTML tags.
Editing a web site whether it is done with a text editor or a an HTML-editing program does ultimately mean altering what is in the HTML and sometimes also the associated style sheets.