DF = Diamond Frame, which is the conventional frame for a bicycle. Upright is another term used for conventional bicycles (because some don't actually have a diamond frame, such as many mountain bikes and folding bikes, but they still have the upright riding position). You may also see "wedgie" (so-called because of the saddle giving you a wedgie) and "upwrong" used as negative terms for uprights.

Here's a recumbent glossary that's worth reading through: http://rbr.info/support/recumbent-glossary.html