Sable is the most durable and soft hair used in makeup brushes. They should be viewed as an investment since they'll last a lifetime if you take proper care of them. There are three types of sable brushes: Kolinsky is the highest quality, then red sable and then just plain sable.
Sable is popular because its brushes are very resilient, they snap back and they have pointy tips. The hair on sable brushes is long with a fine pointed tip and a thick middle so that it naturally creates a conical shape.
Kolinsky Sable is amazingly soft, golden brown and is commonly found in expensive designer brushes such as this Tarte All-Over Eyeshadow Brush ($24, pictured left). It comes from species of mink found in Russia and China and has the longest length of any of the sable categories (2.25 inches).