The Q factor of a bicycle is the distance (measured parallel to the bottom bracket axle) between the pedal attachment points on the crank arms. It may also be referred to as the "tread" of the crankset. 
Q factor is a function of both the bottom bracket width (axle length) and the crank arms. Bottom brackets axles vary in length from 103mm to 127mm. Mountain bike cranks are typically about 20mm wider than road cranks.
A narrower tread is desirable on faired recumbent bicycles because then the fairing can also be narrower, hence smaller and lighter.
It is also claimed that a narrower tread is ergonomically superior because it more closely matches the nearly-inline track of human footsteps.
Though it seems intuitive that a closer tread is superior, it must be remembered that a walking person must put their foot more to the centerline of the body to balance. This is not the case when pedaling a bicycle, where the "steps" are so very close together and balance a non-issue. some experts believe that to reduce torquing of the knee the q factor should be adjusted so that the foot tracks in line with the knee and hip.