(None of this works in IE – use Firefox.)
The data: URI is a type of URI that contains the data that is returned, in the URI itself.
Basic usage: (Paste these into the Firefox address bar!)
data:text/plain,Hello world!
data:text/html,<a href="http://www.google.com">Google it up</a>
data:image/png;base64,R0lGODlhEAAQAIAAAP///wAAACwAAAAAEAAQAAACH4RvoauIzNyBSyYaLMDZcv15HAaSIlWiJ5Sya/RWVgEAOw==

You can also have things longer than what fits in URL in an HTML “src” attribute, which requires a URI; for instance, you can embed images in your HTML without having a separate file, like this:
<img src="data:image/png;base64,
iVBORw0KGgoAAAANSUhEUgAAAJYAAACWCAIAAACzY+a1AAAAAXNSR0IArs4c6QAAAARnQU1BAACx
jwv8YQUAAAAgY0hSTQAAeiYAAICEAAD6AAAAgOgAAHUwAADqYAAAOpgAABdwnLpRPAAABglJREFU
eF7tnethnDAQhJO2riDqoRqauWIcHuJ5IO1I4jwbxr8cexG782n0gnP+/vz8/NGXawV6hPpyrcAf
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BciFQuheAfcFyIVC6F4B9wXIhULoXgH3BciFQuheAfcFyIVC6F4B9wXIhULoXgH3BciFQuheAfcF
yIVC6F4B9wXIhULoXgH3BciFQuheAfcFyIVC6F4B9wXIhULoXgH3BciFQuheAfcFyIVC6F4B9wXI
hULoXgH3BciFQuheAfcFyIVC6F6BqYB3+xr+rOyrfYfv+n80nYvi5MIJYNO+V5KBXdcUQOyaoUsU
NGDuPkI4SNW1E8H+u2Yj+/pjs54bRwshpFph8OzCaRDtR9OAs8REow1LGrDWJBfu5sIR3zQGluov
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Eu/1qqF/jTaKKGb1vLsvqoQwNUiOvd06lVzWPLZyw2i2W6Pe0P69ECsgHARI0Uktz3a/v17/pZop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" />
April 12, 2008 at 9:46 pm
Note that Internet Explorer 8 Beta 1 now supports data: URIs. (Just for the benefit of whoever finds this post in a Google search)
April 13, 2008 at 2:22 am
Thanks, didn’t know.
Do they support CSS 2 and PNG alpha channel yet? ;)