What is "Balun" and its application?

I've searched some articles on Balun. These components seems to find their application in microwave circuits, for example, PA and LNA etc.

Can anyone tell me the function of "balun" and its difference compared with transformers?
At what frequency range are "baluns" used, from lump to distributed baluns?

I would be also glad if you give me some article links introducing these RF components.

Thank you!

Balun , is "balanced unbalanced , this mean this circuit transform the single ended signals to differential and vis versa ,
Balun can be implemented by center taped transformer but have many other implementations line hybrid couplers in microwave circuits and also can be implemented by the microstrip lines

khouly

"What is a Balun, and Do I Need One?"
http://www.cebik.com/a10/ant47.html

Regards,
IanP

thanks too!

A balun is a device designed to convert between balanced and unbalanced electrical signals, such as between coaxial cable and ladder line. Baluns can be considered as simple forms of transmission line transformers.

for more read this page

http://en.wikipedia.org/wiki/Balun