Smolder is MAC's standard permanent black eye kohl, which gives a deep smudgeable line.
Feline is MAC's limited edition black eye kohl called "Kohl Power". The Kohl Powers contain carbon, which gives them a much much richer black base (compared to other MAC pencils), as well as a very emollient creamy texture. They also have great staying power, but can smudge alot, so some people disliked that aspect. As for me it is my FAVORITE pencil ever.
I have like, 7 backups.
Both can and are intended for use on the waterline.