What does the use of the # symbol mean in XML Document Type Definitions?

So in a lecture today we were learning about creating DTDs for XML documents. One of the examples contained the following lines:

<!ATTLIST Order key CDATA #REQUIRED>
<!ELEMENT Name (#PCDATA)>

I asked my lecturer why there was a # before PCDATA but not CDATA, but he didn’t know (he guessed it might be something to do with encoding) so I put it to you, collective: what does the # symbol signify?

I know I could probably trawl for disjointed answers on forums and the like, but I hoped a kindly jelly could tell me straight, or at least point me at a nice resource :D