What is XML for?

I often wondered what is XML for. Where do the ideas come from. Initially I thought that XML was meant for storing data in the presense of an object oriented data store. But I've since discovered that XML is for making forms, you know those forms you have to fill in all the time, and XML document is simply a form that has been filled in.

Once you understand XML in these terms then its design becomes much less of mystery. You get a hint from a form what the process model is behind it, but only a hint. Also, if one has a database then one copies information between the form and the database: either fill out the form from the database, or vice versa fill out the database from the form.