Hi there GotoBrazil.
Subliminal is an adjective and it refers to a communication that is not intended to be understood consciously, but to influence thoughts, feelings, and/or behavior at the subconscious level.
Subconscious can be a noun or an adjective and it refers to processes that take place in the human psyche of which we are not totally aware.
Or if I try to explain In simpler terms I would say that subliminal originates "outside" the mind and affects the mind at a subconscious level. The subconscious is "inside" and a part of the mind (the part to which the subliminal is directed).
Hope this helps.^^