The Helvetica Neue Light is a variation of the Helvetica font which maintains a lot of the characteristics of the original Helvetica. It is simple, minimalistic, round and yet elegant. The Neue variant of the font has these same characteristics. There is only a slight variation in the design of both these fonts and they can be easily used instead of each other.
However, if a graphic designer pays a lot of attention to detail, he will realize that Helvetica Neue is slightly less round than the original Helvetica. This fact can be used for creating a different look in advertising copy and even signage.
Check out: Free Helvetica Font: Helvetica Neue