Rudiment set in direct opposition of the ruff. Drags are more popular because it isn't a logistical nightmare to compare them with other things.

A drag looks like this:

>                > 
R   (ll)   r  |  L   (rr)   l  ||

 |____3____|     |____3____|
 
The firm definition of a drag is an accented note with a diddle following on the opposite hand. I have placed them here in alternating triplet form for clarity.