Q factor
 

I here it but dont know what it means.In english,what is it?

Q factor refers to the length, or space of each crankarm to each other. a smaller Q factor number would mean a narrower space between crankarms, thus the pedals would be closer to the frame (your knees would be closer to the frame) supposedly making for better power transfer and better aero position...

i think that dura-ace has the narrowest Q factor out there.

Former hour- record holder Graeme Obree designed a track bike with a really narrow Q, to make himself more aerodynamic.
His feet were so close together, there was no room for chainstays, so his raised them out of the way.
Generally, modern systems(like MTB triples) have a really wide Q compared to older/vintage models.
If you want to chose a particular width, select it on the basis of hip-width, leg and crank length, and aerodynamic requirements. There are no guidelines, and most people take what thye are given.

q-factor is also known as 'tread'
i think TA cyclotouriste cranks have the narrowest tread.
harris-cyclery has them