Do PLC's use logic gates or Logic Ladders

Hello All, Do PLC's use logic gates or Logic Ladders? I've been asked to explain two different types of stop/start/retain relay control circuits for suitable for either the AC or DC I'm going to compare an electro-magnetic relay system with an electronic logic or PLC system but I'm not sure if I'm getting a PLC and Electronic logic system mixed up or they are both the same? Replies would be gratefully appreciated.