XML is a markup language for documents containing structured information. It is similar to HTML in some way. The difference between XML and HTML is:
In HTML, both the tag semantics and the tag set are fixed. An <h1> is always a first level heading and the tag <product> is meaningless. The W3C is constantly working to extend the definition of HTML.
XML specifies neither semantics nor a tag set. In fact XML is really a meta-language for describing markup languages. One of the greatest strengths of XML is that it allows you to create your own tag names.
XML is a kind of neutral information-wrapping system, which means information can be packaged and transmited and used in a way that's independent of the kinds of computer and software that are involved.
After 10 years development, XML is now widely used, there is essentially no computer in the world, desktop, hand-held or back room, that doesn't process XML sometimes.
Here's a link which contains a good introduction to XML: http://www.xml.com/pub/a/98/10/guide0.html
Just enough XML to survice
Google XML Sitemap
What XML can do for those half-bottle-watered non-techie computer fans such as me or maybe you? One very important job XML can do for us is Google XML Sitemap.
Well, I just need to know what XML is and how it is defined, but needn't actually do it myself. If you have a small site hosted by any free hosting service, you may wish your site to be indexed quickly and thoroughly by Google search engine, do some SEO (this buzz word stands for Search Engine Optimization) jobs! Then you may need submit your sitemap to the search engine. This sitemap file is written in XML language.
If you have a little site created by Google's Page Creator, you think that just because this product is by Google that their site(s) will be instantly indexed in Google search engine. This is wrong because a Google Pages site is treated just like any other site.
It's easy to veryfy your site if know how to use Google Webmaster Tools. But you may be wondering how to submit a sitmap. Sometime it's a little bit tricky, because you can't see any sitemap file under Page Creator file directory. Where does that XML format sitemap, which is required Google, hiding?
But, actually GooglePages automatically creates a sitemap, which is available at: http://your-site-name.googlepages.com/sitemap.xml.
As so, you only need to fill in that url when prompted.
When you finish this, you should have a "1" under sitemap, and a green "v" under "verified?". Good news, everything is fine now! You can now explore all the features given by this powerful tool.