Is XML that different from HTML?

I've written many web pages in HTML, but I haven't read much documentation on XML. I'd like to know why it's supposed to be so much more powerful, and if it's okay (starting a business online) to keep using HTML. I haven't exactly see any .xml pages online anyway.

[Paul Victorey]

At the moment, it's mostly slight changes in syntax.

For example, tags that have no ending tag (for example, the HR tag) must look like this:

<hr/>

Also tags must be lowercase.

The real feature I think is it allows the web page creator to define new markup languages, and then use them. How that works I don't know.

BTW, re: your last message -- your email doesn't work

[mosquito]

Carefull here.
XML is NOT html!!!, nor is XML a replacement for HTML (it isn't and it never will be.)

XML is a system to exchange DATA (so mainly for Business to business communication), the power of XML lies in the fact that you can give the data a name (the "self describing" feature they keep talking about.)
so a text file with the following content

1,test,test description
2,not a test,a crappy description
3,another test,description

can mean almost everything. Maybe the first field is an id number, but it can also indicate a daynumber or anything else...
If you use XML to represent this data you will end up with

<testdata>
  <division id=1>
    <name>test</name>
    <description>test description</description>
  </division>
  <division id=2>
    <name>not a test</name>
    <description>a crappy description</description>
  </division>
  <division id=3>
    <name>another test</name>
    <description>description</description>
  </division>
</testdata>

if you attach a schema file to this xml document which describes the datatype of the tags and stuff you have everything you need to do whatever you like with this data. If you have a legacy system that wants an import file with the format "name", "id", "description" you can transform this xml document with an xsl to this format. If you want to use this data in web pages you can use a different xsl to transform this data to html code.

So xml is just a way to pass data.

Also remember that the < > characters are not XML, they are just used to persist xml data in a text file.

Thanks for your help guys... I think I'm going to read up a little more on XML.

[UncaDanno]

Really, HTML and XML are both subsets of SGML. XML validation is more strict about things such as case sensitivity and matching tags than is the validation for HTML (part of HTML's legacy of being used by "forgiving" browsers).
And yes, XML's true power lies in its use for data transport and the ability to create your own tags.
There are a multitude of industry-related consortiums that are developing schemas targeted for their specific industries. I just came off a contract working with an industry-specific schema.
You can also use XML to control the presentation of the data, although few browsers currently support it. As mentioned above, you can write your own tags. You can also write XSL, DTDs, and XHTML and use them together to control the presentation. Sort of like CSS on steroids.
In fact, different pages can use the same data retrieval mechanism in the background and, using XSL, present the same XML document in different ways in the different pages.

I like it!

[mairving]

XML is one reason to learn to write good clean code in HTML. It is not as forgiving as HTML.
Here is an interesting article about how XML is shaping up.