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...
- a standard must be in place to define what a valid markup is -
In the example above, markup is defined as a bit of yellow
ink atop text. In HTML a markup is a tag.
- a standard must be in place to define what markup means -
In the example above, a yellow highlight means the highlighted
text represents an important point. In HTML each tag communicates
its own layout of formatting meaning.
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
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:
The above HTML-encoded data would be displayed by an HTML processing application as: