C7 and C9 style Christmas lights are the large, traditional lights that you will see on Christmas trees and other outdoor Christmas displays. They were very popular back in the 1950s.
C7 lights are energy efficient. For example, if you are decorating a 6' Christmas tree, using 50 C7 lights will make your tree look bright and festive.
C9 lights are easy to see from outside. Use a C9 on a tree, bush or house display and it will be very visible from a great distance.