In the old days anglers used heavy double tapers to cast the long two handed spey rods. However these are not the best line for producing easy casts, so spey lines started to get developed. Spey lines are usually lines that have a fairly long head length - something in excess of 50 ft, and usually with an integrated running line. There are many different types of spey lines; Short belly lines, mid belly lines and long belly lines, and the aforementioned document "Understanding Spey Lines" covers the differences in more detail.
The advantage a Spey line has over the Scandinavian and Skagit type lines is versatility. Spey lines are a good choice for anglers that are going to fish in multiple destinations, for a variety of species and in all seasons.