Internal Ferrules
Used to connect carbon tubing to make longer spar lengths. Half the ferrule is glued to the inside of a piece of tubing, and the other half is left free to accept another piece of tubing. Allows for easy break down for transport. 
You will need to know the internal diameter of your tube.

External Ferrules
Used to extend fiberglass or hardwood spars into longer lengths, and allow easy breakdown for transport. Half the ferrule is glued to one end of the spar and the other half is left free to accept another spar.
You will need to know the Outside diameter of your tube or rod.