XML stands for eXtensible markup language

It is a communicative language to separate, annotate, emphasize or distinguish text on a page.
*allows us to make own Tags (extentable)
*Tag based syntax like HTML (use of marks (<>) called tags too)

Features:
To structure and describe information
To exchange data between disparate systems

Applications:
Used in HTML
RSS/ATOM
AJAX-asynchronous javascript and xml
web services

For more on XML refer below pages: