Ventilation is the fresh air we bring in mechanically, via fans. There are five things to consider when designed a ventilation system (1) how the system will interface with outside air (2) whether the system includes any method to distribute the air within the building (3) whether there is filtration and (4) whether the ventilation system is integrated with a forced air heating system (5) how the ventilation system is controlled.
Do you need a Ventilation System?
Depending on how tight your building is and how much pollutants are expected to enter it, you may not need a ventilation system at all, although even in moderately tight buildings you are taking some risk if you don't have one. Always keep in mind that the infiltration rate for a building will vary from close to zero in mild weather to close to the ACH50 value in extreme winter weather even though the yearly average will be between 1/15ACH50 and 1/25ACH50 depending on local climate. See the full discussion in the ventilation and filtration section. Another consideration is the energy use tradeoff between air tightening and ventilation, which is covered in the infiltration discussion.
Types of Ventilation Systems
There are three types of whole house mechanical ventilation systems, and a strategy for passive natural ventilation. Unless the house is built extremely tight, it has some degree of natural ventilation during some of the year, that amount may be sufficient to meet the house's fresh air needs during that time. Mechanical ventilation has the advantage of supplying a known amount of fresh air, in addition to be able to provide filtration by circulating the air in the house (although with an energy cost and possibly additional up-front cost). With all whole house ventilation systems, there may be additional spot ventilation, for example in bathrooms and kitchens. If spot ventilation exists, the only difference between it and an exhaust only system is that the control is fully manual.