So, basically, what is XML? A five minute guide.

Although there can't be many people who haven't heard or come across XML in the last year, there is still a surprisingly large percentage of people in the industry who don't fully understand exactly what XML is.  Hopefully this can explain at least the tip of the iceberg.

First, and foremost, XML isn't a one-fit solution for every IT problem that has ever existed.  Having said that, it is extremely useful for the right problem, and it IS making a big difference in the computing industry.

XML is basically a file format, much like a Microsoft Word document, an Adobe Acrobat file, a spreadsheet file, an online HTML page, or a Comma Separated Values file.  However - unlike most of these - XML has the following properties:

• The XML standard is not controlled by a single commercial organisation.  XML is therefore an 'open' standard, which anybody can theoretically contribute towards, or use.
  
  
• Technically, XML files are not stored as 'binary' data, but rather are plain text.  This allows XML a level of platform-independence, and also means that many XML files are human readable.
  
  
• XML files do not have a specific structure; rather, they follow some basic (but strict) rules.  This allows XML files to represent many different kinds of data and information, from documents, to image files, to financial transactions.
  
  
• XML rules can be used to restrict the structure and type of data - therefore new standards can be established.  The structure is also self-descriptive, in that data is assigned a 'tag', so that each item of data is described in some way.  This allows for computer based validation of files, and also some level of human understanding when viewing a file.
  

These last two points are the most important - by defining strict rules, but no set structure, organisations can develop 'standard' XML formats (called Schemas) that represent a particular type of data.  Standard XML schemas exist for many different types of information - from business sales transactions to news headlines.

This can also cause problems - although XML is a standard (i.e. the rules for creating these files), the structures you can create with it are so varied that many organisations are creating similar, but sufficiently different, competing XML schema descriptions.  This initially slowed down the uptake of XML, but organisations are now beginning to come together (many through the W3C) to develop and promote single XML standards for each sector/problem domain.

For a more succinct description of XML, try the 'XML in 500 words' link.  For the other side of the coin, try the 'XML Suck' web site, for interesting views on why XML isn't always right for the problem in hand.