As Freud illustrated in his iceberg theory, the human mind consists of a small (10%) conscious part which we all know, and the larger (90%) part known as the subconcious mind. Between these two parts of our mind, there is a reasoning mechanism known as the Critical Faculty. In Freud's iceberg diagram, it is represented by the waterline. It is often referred to as the gatekeeper between conscious and subconscious thought.

Subliminal suggestions are thoughts, ideas, commands, and other messages that are designed to effectively get through the Critical Faculty and into the subconscious mind.

In psychology, in hypnosis, in NLP (Neuro-Linguistic Programming), subliminal suggestions can be useful and powerful tools for positive change. In advertising (AND THERE IS A TON OF SUBLIMINAL SUGGESTIONS IN ADVERTISING!!!) subliminal suggestions can be flat out manipulative and deceitful. 

Here are some articles on Subliminal Suggestions:

The impact of Subliminal Suggestion on people's choices. This article also discusses the making of subliminal CD's, which we do at Family Mental Health and which can be found HERE. 

Here is an interesting article on the history of subliminal suggestions primarily used in advertising.

In hypnotherapy and self-hypnosis, suggestions are generally overt and known. With subliminal suggestion, the messages are generally covert and unknown to the conscious mind. In both cases, however, the information is by-passing the Critical Faculty and entering into the subconscious mind. The process is defined and explained a little bit in THIS ARTICLE.