here is the image i upload,i dont know what it call,i want to study this circuit,but dont know how,i know it is just a diodes characteristic,but some how im still not understand.
is that have a term or name for this particular circuit?

 

Hmm
http://en.wikipedia.org/wiki/Diode_bridge
http://www.allaboutcircuits.com/vol_3/chpt_3/1.html

 

This is a diode bridge. 4 diodes are arranged this way in a bridge rectifier.