Ventilation is the main factor in ensuring that an indoor environment of any building - may it be a school room, an office space, or your home - is comfortable for any person and occupant inside. There are two main forms that most systems are categorized by: either natural or mechanical.
Natural ventilation gets its name from its reliance on the natural flow of air. Like how a fluid is any substance that has the ability to flow, the air around us is one of those substances. Thus, natural ventilation refers to the fluid nature of the air, which includes the differences in pressure on the outside and inside of a building, and within the interior of the building, as well.
Natural ventilation puts a heavy reliance on a process called infiltration. Infiltration refers to how air tends to escape through routes within a building to the outside, whether it is through open windows or some other similar method. This can pose a problem for - let's say - when in the winter time, you want to keep the warm air contained within your home, but it's constantly escaping through those routes we talked about. This can cause you to use even more energy than necessary to warm your living space, since you're replacing the warm air being lost and constantly fighting against the cold air. Needless to say, this directly impacts your energy bill, and we don't want that.
Mechanical ventilation is basically forced ventilation. It bypasses the problems we run into with natural ventilation by withdrawing the stale, sitting air within a space, and exchanges it with a steady flow of crisp, fresh air. Because of its design, it does all this noiselessly and dependably - most times, just at a the push of a button.