Plain-language answer: A balun interfaces balanced and unbalanced systems; an unun transforms between two unbalanced systems. Either may also transform impedance, but the name alone does not state ratio, loss, or choking ability.
Why it matters at the radio
A transformer chosen only because its connectors fit can create the wrong impedance transformation or fail to control common-mode current.
What to watch in the field
Identify balance on both sides, required ratio, frequency range, power, duty cycle, and measured common-mode impedance before choosing.
The common misunderstanding
Calling every transformer a balun or assuming every 1:1 balun is an effective current choke.
Learning boundary
This lesson explains established radio and antenna practice. Current regulations and the exact equipment or antenna guide control legal limits, ratings, compatibility, and safety.
