### Plain English Answer
Hi fletch,
You asked for "simple and jargon-free English". Right off the start you
are
in for a challenge, because that's not easy to find (for any
technology).
Here then, is about as simple and jargon-free as you will find
anywhere, and you can come back to this as you progress in experience
with all the uses, practices, annoyances,
and challenges that face you in the future. ...
Ready? Here goes:
1. XML is one way to make it easier to write computer programs that
output and input data.
2. XML is one way to make it easier to indicate the different parts of
any kind of message, in a way that a computer (or rather, a person
writing programs that run on a computer) can easily understand without
making mistakes, and without unwanted ambiguity.
3. XML is not the only way to do these things, but it is a popular way,
and it is one of the few ways that can be done using tools no more
sophisticated than a simple text editor and your favorite computer
programming language.
Apart from this, there are thousands upon thousands of ideas, opinions,
preferences, wars, rumors, debates and innovations that have "XML"
somewhere in the name. In the end, the main "why" that may distinguish
XML from any of its alternatives (and there are many) may be:
4. XML is respected enough to justify putting on your resume; complex
enough to support many different kinds of ideas; broad enough to
support an entire ecosystem of jargon and hype (essential ingredients
for any technology ... if it is going to make money or gain momentum);
and productive enough to offset the time and cost invested in learning
it (at least according to many folks out there).
### Your Homework
And now for your homework:
1. Do the following search on Google (copy it exactly):
site:microsoft.com "what is xml"
2. Read the following article to keep the "hype" in perspective:
http://www.well.com/~doctorow/metacrap.htm
This is more than enough to get you started.