Surely, if you have decided to learn about XML, you are probably already quite familiar with the concepts behind HTML (HyperText Markup Language). So let's start from there.
HTML, as its name implies, is a markup language. As such, it is used to markup text. But what exactly does it mean to markup text?
Abstractly, marking up text is a methodology for encoding data with information about itself. Examples of markups (encoded data) are ubiquitous in the real world.
For example, back when you were slogging through high school, you probably used to use a bright yellow highlighter pen to highlight sentences in your schoolbooks (or at last you knew someone who did!). You did so because you thought that the highlighted sentences would be useful to review around exam time and you wanted a quick way to skim through the important points. Just like you, thousands of kids around the world did the exact same thing for the exact same reason.
By highlighting certain bits of text, you were effectively "marking-up" the data. Essentially, you specified that certain sentences (data) were important by marking them in yellow. These sentences became encoded with the fact that they were important.
And what's more, since everyone followed the same standard of marking up, you could easily pick up a used text book and get a good idea just from reading the highlighted sections what were core points of the book.
There are two crucial points to take away from this example. For markups to transmit useful information about data to a pool of users...
Markups are also ubiquitous in the world of computers. They are used by word processors to specify formatting and layout, by communications programs to express the meaning of data sent over the wires, by database applications that must associate meaning and relationships with the data they serve, and by multimedia processing programs which must express meta-data about images or sound.
As data is sent through dumb computers and programs, it is essential that the data carries with it information necessary to communicate what the data means and/or what the receiver should do with that data.
Data with no context is meaningless just as an unhighlighted book is bad news around exam time!
HTML is one of the more famous computer markup systems. HTML defines a set of tags that associate formatting rules with bits of text. Documents which have been marked up (which contain plain text as well as the tags that specify the rules for formatting that text) are read by an HTML processing application (a web browser for example) that knows how to display the text according to the rules.
For example, the <B> tag specifies a rule which instructs an HTML processing application to bold a specific bit of text. Similarly, the <CENTER> tag instructs the HTML processing application to center the text.
Thus <CENTER><B>BOLD</B></CENTER> would be displayed by an HTML processing application as
You might imagine a client contact list which could look like the following bit of HTML code:
<UL> <LI>Gunther Birznieks <UL> <LI>Client ID: 001 <LI>Company: Bob's Fish Store <LI>Email: firstname.lastname@example.org <LI>Phone: 662-9999 <LI>Street Address: 1234 4th St. <LI>City: New York <LI>State: New York <LI>Zip: 10024 </UL> <LI>Susan Czigonu <UL> <LI>Client ID: 002 <LI>Company: Netscape <LI>Email: email@example.com <LI>Phone: 555-1234 <LI>Street Address: 9876 Hazen Blvd. <LI>City: San Jose <LI>State: California <LI>Zip: 90034 </UL> </UL>
The above HTML-encoded data would be displayed by an HTML processing application as: