Proper attic ventilation is one of the most important, yet least understood systems in a house. On a regular basis, I inspect homes with venting problems that range from mild (not yet causing a problem). . .all the way to severe (deterioration of the rafters and roof decking).
Attic ventilation is almost always passive in nature, and relies on convective air flow, where cooler air is drawn into soffit vents along the eaves and rises through the attic and passes out the roof vents on top. Both soffit and roof vents can easily become blocked, and should be inspected on a regular basis.
The recommended minimum attic ventilation rate is one square foot of roof ventilation (one typical roof vent provides approximately one square foot of ventilation) for every 150-200 square feet of attic space. . .remember, as far as attic venting is concerned more is almost always better!