Q-factor is the ratio of the center frequency (f0) to the bandwidth.
High Q-factor transducer indicates a narrow bandwidth and a long spatial pulse length with low damping and narrow bandwidth. This commonly used in doppler ultrasound where a narrow bandwidth is needed to accurately quantify flow rate.
Low Q-factor transducer indicates a broad bandwidth and a short spatial pulse length with high damping and large bandwidth. This is common in most imaging applications where a short spatial pulse length is needed to optimize spatial resolution along the beam axis.