Be sure you read http://www.xml.org/xml/xmlfaq.shtml and the various other FAQs that are linked from that document.
Billy, there are two types of valid XML. One is "well-formed," which means simply that the document follows the rules for writing XML. That covers such issues as lower-case tags, backslashes in non-container tags such as <BR >, proper nesting of nodes, et cetera.
The other type is conformant XML that maps to a DTD. Such XML can be validated -- i.e., tested against the DTD. A DTD is a machine-readable document that describes the legal usage of various tags.
For "standard" DTDs, there may be associated documentation that describes the purpose of a tag, which is not the same as its legal usage. (For instance, <partnumber></partnumber> in a DTD for ecommerce might be explained in a document.)
There are as many DTDs as there are DTD authors.
Now, to get to your specific case: It appears that you are dealing with documents. There are many document DTDs, and most of them are so complex that it's not worth your time to get involved in them.
For instance, NITF is designed for news organizations such as wire services to transmit structured news documents:
http://www.nitf.org/
If you actually try to implement NITF as your first XML project, you probably won't come out alive. :-)
But if you look at NITF, you'll discover that the authors of that standard chose to follow conventions similar to HTML and XHTML for the structural markup of text.
If we already have a well-accepted <p> tag, then it doesn't make a whole lot of sense to use <para> or <paragraph> instead.
I would suggest that your best bet is to look closely at XHTML, which is a slightly cleaned-up version of HTML 4.0, for guidance as to how to mark up structure within text blocks.
Hope this helps.
