What's the difference between the positive pressure and negative pressure?

positive pressure is when pump suction flooded with water means pump will get the water on suction.

negative pressure when pump suction fare away from water source and have to use other support method to get water from tank as example when tank is in lower level than pump 

It depends on where it is used, in fire fighting we use the term positive & negative pressure in the way we ventilate, in this case to use positive pressure is to push air into the building/ structure to prevent smoke & fire products from moving into a certain area like the rescue stairs, and we use smoke extrication fans to pull smoke outside a structure if we decide to use negative pressure ventilation. I hope I have made things clear for you Yousef. Good day, 

Positive pressure is more than Atmosphere pressure & negative pressure is vacuum pressure.