pass-transistor logic or pass-gate logic are a class of non-restoring logic families that use transistors as switches such that the output logic levels directly come from the input. This is as opposed to the logic that connects the output node directly to Vdd or GND.
PTL makes use of passive devices carrying the input to output without farther amplification. This produces logic with reduced number of active devices thereby producing lower overall transistor count and a reduction in associated interconnections. The obvious drawback of this is that with each cascading device, the signal swing is reduced. For this reason, restoring logic must be added to restore the logic levels.