Negative Pressure Ventilation
Mammals like humans use negative pressure ventilation as opposed to amphibians like frogs that use positive pressure ventilation. Frogs gulp the air and force it in down their throat to breathe. Even though this is inefficient, gas exchange also happens through their skin. Mammals use negative pressure to suck air in. The diaphragm contracts and move down, the intercostal muscles contract and move up and out. This causes the lungs to expand. The pressure inside the lungs drops. The negative pressure sucks air in. For us humans, negative pressure ventilation is the normal physiologic way we breathe.
Positive Pressure Ventilation
Positive pressure ventilation is mechanical. Instead of negative pressure created by the contraction of the diaphragm and intercostal muscles pulling air into the lungs to fill the alveoli, you use either a breathing machine (ventilator) via ET tube or non-invasive bag-mask, CPAB, BiPAP, etc to force air into the lungs, expand the lungs and deliver oxygen to the alveoli.