What goes on threads first: tape or dope?
If you're using both, the tape would go on first. Which is immediately evident, if you've ever tried to apply them the other way round.  There's open debate over whether or not there are any advantages to using both products together, or if each product by itself is adequate.
Pipe dope
Pipe dope; also known as pipe joint compound, is used to seal and lubricate the threads of a threaded pipe joint.  It's available for both water and gas lines. It's applied by simply brushing it on the male threads of the pipe joint, before the joint is fit together. Pipe dope never hardens, and will not crumble and fall out of the joint.


Teflon Tape Dope
Teflon Tape dope; also known as polytetrafluoroethylene (PTFE) Tape, or thread seal tape, is also used to seal and lubricate the threads of a threaded pipe joint.  It's available in high-density and low-density flavors, for water and gas pipe.  PTFE tape is applied by wrapping it in a clockwise direction, around the male threads of the threaded joint. PTFE tape is available in color coded versions (pink for water, yellow for gas), as well as the common white variety.