First, let's look at the kinds of roof vents.
Louvers are covered openings that allow air to escape the attic. They are located on the gable (non load-bearing) ends of the house and allow limited airflow. This system relies on wind direction to create airflow through the attic. When the wind blows perpendicular to the ridge, it circulates air around the louver, which acts as it own intake and exhaust vent. If the wind blows parallel to the ridge, the airflow pattern acts as a draft that moves in one louver and out the other.
Fans draw air through the attic by creating an air flow. They are effective, but they don't allow natural forces to ventilate the attic. Plus, it's somewhat expensive to buy, install and operate power fans, a cost that is greater than the fans' benefit. Fans do not create airflow over the roof sheathing. Even fans that automatically turn on and off at selected temperatures are not worth the investment because the summer heat in the attic will cause the fan to run almost continually.
Mounted along the top of the ridge, ridge vents provide even, consistent exhaust ventilation. Run a ridge vent the entire length of your attic. Some roofers cut costs by running only the length required by code. Rosie recommends running the vent from roof end to roof end for best results. A ridge vent won't work well alone; adequate intake ventilation must accompany the ridge vents in order to maintain airflow.
A common method of ventilation, soffit vents are located under the eaves, where they will have minimum exposure to rain and weather. Soffit vents on each side of the structure create equal ventilation on both sides. Soffit vents should not be your home's only vents, though. If they are, the air movement is restricted to the attic floor, and air flow does not pass over the roof sheathing.
Also called whirly birds, turbine vents are wheels mounted near the ridge of the roof. You generally install two or three per roof. Most of the time, they act as exhaust vents, but air also can enter the attic through a turbine. Turbine vents are not effective, as they only exhaust small portions of the roof, and they allow rain to enter the attic.
Rosie's recommendation: Ridge vents accompanied with soffit vents will create the most effective attic ventilation for your home. The combined system will draw air from the soffits along the sheathing and exhaust it through the ridge vent.