i think the title speaks for itself.

 

The three angles are ground or cut into the valve seat in the head. Typically, a 45 degree seat face with a 15 degree top cut to blend into the combustion chamber and a 60 degree bottom cut to blend into the the valve bowl ( the area in the port side above the valve ). The angles may vary slightly to allow better airflow though the engine. A three angle seat allows more flow at low valve lift, while retaining good flow at higher valve lift due to the radius effect of the combination of angles.
The valve may also get ground with what is known as a "back cut". This the removal of material above the seating area on the valve, and also gives better low valve lift flow.
A properly machined valve seat will have less than 0.001 inch total runout.

 

Very well said CaptNemo. Just to add a little more, the reason for increasing the air flow is to keep the valve and seat cooler, increasing the life of the valve and seat - this is done by decreasing the contact surface area, which in return reduces the friction heat. So a 3 angle valve job not only helps performance, but also helps with the life of the engine.