So, what is it used for?

XML solves your problems

XML stores your data

XML makes the coffee

XML gives meaning to your data

The key point is this last one. Meta-Data is your friend. That's the door open to true inter-operability and re-usability at low costs.

Another key concept is the clear separation between the data and the way it is rendered or displayed. In XML you put the data and any meta-data to identify this data. Then on a stylesheet you will define the way a particular type of data is rendered.