What the difference is between an electrofusion & a transition fusion coupling of hdpe pipes?

Electrofusion for HDPE pipes are used when connecting two HDPE pipes together. They are also used when you want to reduce the size of a pipe by using an Electrofusion reducer.

Transition Fusion is used when you want "transition" from HDPE to another material, such as Gunmetal or Brass connectors. One end of the Transition Fusion adapter is HDPE, which you can connect to by Butt Fusion, while the other end is the material to be transitioned to, such as a brass connector. 

I hope this helped.