A half wave rectifier is one diode, the half of an AC waveform which is above 0 volts will be conducted through the diode .The negative half cycle will not.
A full wave (bridge) rctifier is usually* 4 diodes arranged so that both positive and negative half cycles will be conducted resulting in both half cycles appearing on the output as positve voltages.
I'm sure if you search the web you will find nice diagrams of this.
(* A centre tapped transformer and 2 diades can also produce full wave rectification)
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