Semiotics and subliminal advertising are two distinct communication mehtods that attempt to persuade consumers into buying, using or thinking about their product. They are two different techniques yet they are often used in together.
Semiotics deals with symbols in the form of text and pictures, and also with their underlying meaning.
Semiotics is a powerful tool for revealing concealed meanings in Internet marketing. Internet marketing makes extensive use of symbols in order to affect product awareness and cultural meaning, semiotics is a very effective and efficient tool for interpreting online marketing.
Subliminal advertising attempts to subconsciously alter preferences or attitudes toward consumer products’. It Exposes consumers to product images, brand names, or other marketing stimuli without the consumers having conscious awareness. Once exposed to a subliminal marketing stimulus, the consumer is believed to decode the information and act upon it without being able to acknowledge a communication stimulus’. They have been inserted into advertising for centuries with great success. Subliminal advertising is illegal in the U.S. and U.K.
Watch the video below.

Other interesting links about semiotics and subliminal advertising
http://www.umsl.edu/~jansonma/myarticles/D__SIGPHL02-1364.pdf
http://www.youtube.com/watch?v=lLaQJ_Kr0UM
http://www.marketingsemiotics.com/expertise/signscape.html