Internally threaded Barbells are called that way because the bar has smooth ends with receiving threads tapped into the end of the bar. In internally threaded jewelry, the bead has a receiving tube machined into it, with a threaded post extending from the center, which mates with the threaded receiving tube on the bar.
Internally threaded body jewelry is arguably better then externally threaded jewelry (causes less trauma to a piercing, much more comfortable to insert, less likely to carry germs or bacteria into a piercing), although it might be more expensive.
We recommend buying internally threaded jewelry as it's better for your piercing and is only made out of certified top quality materials.
14 gauge internally threaded barbell 
Internally threaded curved barbell with cones, 14 ga 
Internally threaded curved barbell, 12 ga 
Internally threaded curved barbell, 14 ga 
Internally threaded curved barbell, 2 gauge 
Internally threaded curved barbell, 6 gauge 
Internally threaded labret, 10 ga 
Internally threaded labret, 12 ga 
Internally threaded straight barbell, 4 ga 
14K Gold Labret internally threaded with Genuine Black Diamond, 14 Ga 
14K Gold Labret internally threaded with Genuine Black Diamond, 16 Ga 
14K Gold Labret internally threaded with Genuine Blue Diamond, 16 Ga 