The circuit $($a$)$ passes only the positive portion of the AC signal.

The output of circuit $($a$)$ is exactly the same as its input.

Circuit $($b$)$ passes the positive portion of the input AC signal and converts the negative portion of the input signal between W and Z to a positive voltage between X and Y$.$

Circuit $($a$)$ is a half$-$wave bridge rectifier.

Circuit $($b$)$ is a full$-$wave bridge rectifier.

Both circuits can be used to convert AC signal to a DC signal.