HTML is an acronym for HyperText Markup Language.  

All Internet Browsers. Internet Explorer, Chrome, Safari, Opera, Firefox, etc, read the HTML of your web site and interpret it to display the page in a browser window. Each browser variety will intepret the HTML slightly differently, resulting in your web page looking a bit different on different browsers. Adding to the issues with browsers are the users that have not updated their browsers to the latest version.

HTML5 is the latest version of HTML and it has greatly increased the number of features that HTML can represent.  Not all browsers can interpret all of the features of HTML5.  

You do NOT HAVE to know HTML to use Joomla.  The Text Editors are made to appear to be something like a word processor, where the contributor can enter text and never see the resulting HTML. As you type into the text box of a text editor, it is modifying your input into HTML tags.  If you click on Toggle Editor, you will see pure HTML text with the tags. Joomla's PHP interface then places this chunk of HTML created by your Text Editor and places it inside the HTML document that is sent to the browser.

Adding to the mix are Stylesheets, or CSS. What these do is tell the browser how you want modify the appearance of the tags. For instance the default color for heading text is black, you may want it to be a dark green. You can change fonts, text size, indentation, add borders with CSS. You can change these things with Text Editors as well, but then you have to change it for every single instance of that heading text.  With CSS, you can change the settings globally, so that every time you have heading text, it will be dark green and you don't need to change anything in the Text Editor.

It can benefit you to learn some basic HTML, as Text Editors are known for doing some odd interpretation of your content, producing results of which you may not be completely pleased. Also, HTML can effect SEO. Overuse of heading tags, improper <alt> tags, can hurt your standings for Search Engine results.

Basic HTML used by Text Editors

Text tags are:

<p></p> Paragraph tags. The text in here is for the paragraphs of content for your page. Each separate paragraph should get its own set of <p></p> tags.
<h1></h1>
through
<h6></h6>
Heading tags used for the heading of the page and the heading of different sections of the page. <h1> bieing of the highest importance.  These heading tags are usually larger and set to be bold, wiht <h1> being the largest.
<ul><li></li></ul> Unordered List, with bullets at every <li> tag
<ol><li></li><ol> Ordered List, with consecutive numbers at every <li> tag
<table><tbody><tr><td></td></tr></tbody></table> These tags create a table structure of rows and columns.

 

Note that for text tags the Text Editor will create a beginning tag, <p>, and a closing tag </p> on each end of that section of text.

By default, browsers will dipslay these text tags according to their importance, with Heading tags being the most important and larger and often bolder.

Other tags you might see when you toggle the editor are:

<img> Used to insert images.  It does not get a closing tag.
<div></div> Used to separate areas of content for special styling
<a></a> These tags are used for links to other web pages. Inside the opening <a> tag will be an href="/" which holds the URL of the web page it is referencing.
   

 

Attributes

In addition to the tag, there are what is called Attributes.  These are things often placed within the tag that tell the browser special instructions for that tag. Other Attributes are tags by themselves that modify the appearance of text. Some typical Attributes are:

alt="  " This is found inside of an <img> tag. Inside the quotation marks is a short description for the image that will be displayed on the screen if the image can't be found.  It is alos used by text readers for people with visual disabilities.  Another use of alt tags is Search Engines looking for images on the internet.  The search Engines don't know what the image is unless you tell them in words.
<strong></strong> This is placed on either side of text that you wish to be bold.
<em></em> This will placed on either side of text that you wish to be italicized (slanted)
 <span></span> The <span> tag is used to apply special styling to a section of text
<br/> This is a break tag.  It is used to  create a new line, but stay within the <p></p> tags.  It can also be used in headings, lists, and table rows.
title="  " This can be placed inside of <img> tags.  It can be utilized by certain JavaScript functions and it will appear as a small box if the user hovers the cursor over the image.
class="  " This designates a class for whatever tag it is placed into.  It is used by the CSS to style that particular item. Most tags can hold the class="" designation, <div>, <img>, <h1>, <p> etc.