Why XML is so popular???

xml is larger than flatfiles, but now that harddrives are so cheap, memory is less expensive and networks faster, the extra size associated with XML in most cases is not an issue.
The benefits of XML over flatfiles is that you can have sub records easily, and you can add in extra details without having to rewrite programs.

e.g.
imagine a csv with order details
a line might contain company, address, order number, quantity, weight
with a flatfile you would either have to encode a type for each line, or duplicate data per line
company 1, addressline 1, address line 2, order number1, quantity, weight.
company 1, addressline 1, address line 2, order number2, quantity, weight.
company 1, addressline 1, address line 2, order number3, quantity, weight

or something like
Company Line,company 1, addressline 1, address line 2
order line,order number1, quantity, weight.
order line,order number2, quantity, weight.
Company Line,company 2, addressline 1, address line 2
order line,order number1, quantity, weight.
order line,order number2, quantity, weight.

as soon as you put this into xml, you can explore the data in a tree structure.
You have an easy to see hierarchy.
You can add extra data knowing that any changes will not cause problems with your customers programs. e.g. I could add in pallet weight on each order, or sub details under quantity, and their old systems would still work, but when they decided to, they could pull in the new details.

The format of a document can be saved easily, and all new documents can be compared against it.
As tags are balanced, it is easy to see if the file is corrupted.
You can mount an xml document, and query it like a database.

The file can be read by a human. This may sound odd, but if you work in a company where you are often asked to check files, the ability to open it up, then search for a tag can save hours of work compared to trying to work out what is wrong with a binary file.

Once you have set up objects in C# you can export details in XML. This means that as you code, you are just filling in object details, will all the joys of Visual Studio error handling, you then make a simple call to save the file down. With this you know your structure will be 100% correct.

You can query xml files with LINQ once you have loaded them. (I have never done this, but have been told it)

The list goes on, but there are some reasons to start you off.

XML - Xtensible markup language. It is used for Exchanging data  either over  Web Application or Windows Application (StandAlone Pc).

It is Plateform Independent

One more thing aboout it that it exchange the message dynamicaly and html is used for statically.

Xml is supported by ado.net but ado does not support Xml

It also forms the basis of exchanging data within applications running on the same machine or on different computers .Web services can be used for that and note that they use XML to do that