Psychologists suggest that the brain works in three ways, which are unconscious, subconscious and conscious.

Unconscious / unoconoscious / ( n-k n sh s) / adj./ 1. Lacking awareness and the capacity for sensory perception; not conscious. 2. Temporarily lacking consciousness. 3. Occurring in the absence of conscious awareness or thought: unconscious resentment; unconscious fears. 4. Without conscious control; involuntary or unintended: an unconscious mannerism.
Subconscious / suboconoscious / (s b-k n sh s) / adj./ 1. Not wholly conscious; partially or imperfectly conscious: subconscious perceptions.
Conscious / conoscious / (k n sh s) / adj. / 1. a. Having an awareness of one's environment and one's own existence, sensations, and thoughts. b. Mentally perceptive or alert; awake.. 2. Capable of will, thought or perception. 3. Subjectively known or felt.

These three ways in which the brain works have to do with the three types of subliminal messages, which are visual stimuli, accelerated speech, and embedded images.

Visual Stimuli: this type of subliminal messages is known commonly as flashing pictures inside a movie or TV show. These pictures or messages are flashed so quickly (from 25ths of a second per frame to 1/1000ths) that the viewer doesn't notice he sees it, but his brain does. In other words, he perceives it without noticing with the unconscious mind.
Accelerated Speech: this type of subliminal message is used by placing music over a low volume dialoged message. This works in a way of visual stimuli, although the viewer, or better yet listener, can be technically more aware of the messages (in a visual stimuli, one may not see the picture even if one tries to and in accelerated speech, one can hear the message if one really tries to). Technically, the accelerated speech is a subconscious way of using subliminal messages.
Embedded Images: this type of subliminal messages is used a lot. In advertisement and publicity, pictures are “camouflaged” in a way that you have to really focus on the picture in order to see them. One may realize they are there by a quick glance, which is the whole idea of this subliminal message. Sometimes people even look back after their quick glance to see if what they think they saw was really there, so in a way, even if you're not paying full attention, you are drawn to these ads. These are considered conscious/subconscious subliminal messages.