Zoning glossary / reference?

Does anyone know of a simple up-to-date explanation of the various zoning terms that are used in Durham? For example on the durham.municipal.codes site, I was trying to figure out what "PDR" meant and here it says it means "Planned Development Residential", but here it says it means "Planned *Density* Residential". And neither really clarify what it *means* if something is zoned that way.