The Ridge of a Roof
The ridge is at the top of a roof where the sides meet to form an angle. The ridge runs the length of the roof, and must be shingled (or capped) with ridge cap shingles to keep water from making it's way into the attic.
Ridge vents (vented roof caps) are often installed to help with ventilation of the attic, and may run the length of the ridge. These vents allow warm air to escape the attic which draws cool air in through vents in the soffit. An important part of keeping the the attic temperature nearer the outside temperature ridge vents help protect against moisture buildup, ice dams, etc...
Ridge vents are normally covered by the ridge cap shingles to maintain a consistent appearance. A ridge vent is visible in the top picture to the right. It can be seen as the dark stripe (or small gap in the shingles) near the top of the roof.
See more parts of a roof described.