Ventilation of patients is almost universally mechanical ventilation (with a ventilator), though mouth to mouth ventilation as part of CPR also falls under this umbrella. Generally there are two modes of artificial ventilation: negative and positive pressure.
Negative pressure ventilation (NPV)
By generating negative pressures around the trunk, ventilation can be induced by the subsequent change to intrathoracic pressure relative to atmosphere. This is the principle of an iron lung.
Positive pressure ventilation (PPV)
By far the most common method is the use of positive airway pressures to inflate the lungs. In the 19th century the use of PPV was discouraged but we have since moved past that.
Discussion of mechanical ventilation can be found elsewhere
Non-invasive positive pressure ventilation (NIV)
Ignoring mouth to mouth ventilation, the bag mask is the simplest.
NIV can be done manually, or with a ventilator; generally a mask, with or without airway adjuncts such and an oropharyngeal or nasopharyngeal airway or laryngeal mask is used
Invasive positive pressure ventilation
The distinction between invasive and non-invasive airways is somewhat arbitrary. The invasive category includes endotracheal tubes and tracheostomies.