if you only have sky (not Sly+) there is only a single cable that comes from the satellite, but once you get Sky+ fitted, there needs to be another cable that comes from the satellite. This is so that You can Record and Watch at the same time. One Cable Records what you want and the other lets you watch another sky channel.

Thats the way i undestand it as my mate works with sky and was asking him what was needed for to do it myself.