The mild hybrid is still a hybrid car that uses battery power and electric motors to improve range and power.
A mild hybrid usually doesn't have a plug in option, and relies on the cars regenerative braking systems to recoup lost energy for later release as power.
The battery is smaller so it doesn't offer much of a range, but can significantly improve fuel economy in stop start round town driving. Often the electric motor is used to get the car going, rather than put the pull and strain on a petrol engine using precious fuel to get the car rolling. Then when rolling the hybrid system cuts in.
A mild hybrid battery is typically only 0.5 Kwh whereas a plug in hybrid can range from 9 to 15kwh with a full electric car battery between 50 and 100 kwh (all typical figures, some models can vary quite wildly from this.)