You want your home to be well-insulated to reduce the cost of heating and air conditioning. You also need the proper ventilation to ensure that moisture and heat can escape from the attic to avoid problems with mold or ice dams. Ridge vents (also called roof vent systems), working in combination with soffit vents, are the key features that provide your attic with the ventilation you need.
Running along the entire roof at the peak, a ridge vent is designed to blend with the roofline to maintain the attractive look of your home. An efficient roof vent system should include an external baffle and an internal weather filter. Working together, they help draw air out of your attic and keep weather from entering your attic. When combined with the soffit vents under the eaves, you have an efficient air-handling system.
Ridge Vents, Soffit Vents & Good Air Flow
In order for these vents to provide you with adequate ventilation, you also need to avoid blocking air flow in the attic. With my handyman business, we often found homeowners with piles of boxes blocking air flow which led us to explain how ventilation is critical to protecting their home.
Note: Older houses may not have ridge vents, as they're a fairly new building code. Ridge vents replace the gable vents you're used to see on the “gable” ends of a house.
Photo credit goes to BuildingScience.com, and here's a Crash Course on Roof Venting.