What is the difference between frequency wild AC system and other AC systems?
Frequency wild Unregulated AC is generated using alternators.
They were more common before constant speed drives were added to aircraft beginning in the 50's.

The alternator runs at various RPM (which varies the frequency) since it is driven by the engine.
— Wikipedia

The difference is that regulated AC is of fixed frequency. Better for motors, pumps, etc.
Unregulated AC was used for things like windshield wipers and windshield heating. DC inverters were used when regulated AC was needed.

Edit: On a plane like the ATR 72, it is indeed referred to as AC Wild.
It is used for heating the windshield, propellers, and probes. Among other systems that don't require regulated AC.

(Source)

ATR notes that discuss AC Wild can be found here. (Courtesy of FreeMan.)