From http://www.w3schools.com (Copyright Refsnes Data)

XML CDATA

All text in an XML document will be parsed by the parser.

But text inside a CDATA section will be ignored by the parser.

PCDATA - Parsed Character Data

XML parsers normally parse all the text in an XML document.

When an XML element is parsed, the text between the XML tags is also parsed:

<message>This text is also parsed</message>

The parser does this because XML elements can contain other elements, as in this example, where the <name> element contains two other elements (first and last):

<name><first>Bill</first><last>Gates</last></name>

and the parser will break it up into sub-elements like this:

<name>
   <first>Bill</first>
   <last>Gates</last>
</name>

Parsed Character Data (PCDATA) is a term used about text data that will be parsed by the XML parser.

CDATA - (Unparsed) Character Data

The term CDATA is used about text data that should not be parsed by the XML parser.

Characters like "<" and "&" are illegal in XML elements.

"<" will generate an error because the parser interprets it as the start of a new element.

"&" will generate an error because the parser interprets it as the start of an character entity.

Some text, like JavaScript code, contains a lot of "<" or "&" characters. To avoid errors script code can be defined as CDATA.

Everything inside a CDATA section is ignored by the parser.

A CDATA section starts with "<![CDATA[" and ends with "]]>":

<script>
<![CDATA[
function matchwo(a,b)
{
if (a < b && a < 0) then
   {
   return 1;
   }
else
   {
   return 0;
   }
}
]]>
</script>

In the example above, everything inside the CDATA section is ignored by the parser.

Notes on CDATA sections:

A CDATA section cannot contain the string "]]>". Nested CDATA sections are not allowed.

The "]]>" that marks the end of the CDATA section cannot contain spaces or line breaks.