App note from Coilcraft all about Balance to Unbalance or Balun. Link here (PDF)
A symmetrical balanced line has two conductors having equal current in opposite (differential) directions. Twisted pair cable and ribbon cable are examples of balanced line cables. An asymmetrical unbalanced (single-ended) line has one conductor connected to ground, such as coaxial cable.
When a balanced source is to be isolated from an unbalanced load, the impedance matching transformer is referred to as a balun. If the impedance of the load matches that of the source, impedance matching is not required, and the balun has a 1 :1 impedance ratio. When the load impedance is mismatched to the source in a 1 :N impedance ratio, a 1 :N impedance ratio transformer is required.