Spontaneous or controlled mode
A spontaneous mode of mechanical ventilator is one that only delivers support to the patient's breathing, if the patient has some respiratory effort. The ventilator then provides additional positive pressure during inspiration in order reduce the work of breathing and improve gas exchange.
A controlled mode of mechanical ventilator is one that delivers ventilation regardless of the patient's respiratory effort. This can be volume or pressure controlled (see below).
In general, as a patient arrives in ICU, a controlled mode of mechanical ventilator will be the initial mode. Examples of this is SIMV.
Pressure or Volume controlled
The choice between these two modes of ventilation is often a matter of clinician preference, as there is no evidence that either mode is superior to the other in relation to any clinically important outcome.
If pressure controlled mode is chosen, the inspiratory pressure is set. You have to then note what Vt is being delivered to the patient to make sure that it is the Vt that you want to deliver.
If Volume controlled mode is used, the Vt is set. Here the effect on airway pressures has to be noted to make sure that the airway pressures are within safe limits.