Resin is from the trichomes, it is just cannabis oils really. Tar on the other hand is the residue left over from the burnt material, whihc contains a fraction of actual psychoactives.. It seems to be a common misconception that any brown gunk after smoking is resin, and not residue.
I've always associated tar biuldups to a bad roll, i could take one bud and roll one that tokes fine, the other that is all drippy. I have found that when i tear my roaches, if i do not burn off the torn edges, then they will quite often drip with the stuff. Not nice.