>is there a good glossary of terms somewhere?
Yes. There's the NMR Monument Type Thesaurus:
http://thesaurus.english-heritage.org.uk/thesaurus.asp?thes_no=1
And for a more in-depth description of monument types, there's the Monument Class Descriptions:
http://www.eng-h.gov.uk/mpp/mcd/mcdtop1.htm
Baz