Short-Term Memory (STM) is a memory system where information is briefly held while it is being used; as a result, it is sometimes called working memory. According to research conducted by George Miller (1956), short-term memory is able to contain approximately seven items or pieces of information at a time and can hold these pieces of information for around 12-30 seconds on average.
Long-Term Memory (LTM) is a memory system where information is placed for long-term or permanent storage. Long-term memory capacity is virtually limitless; the problem is finding a way to actually retrieve the information from memory. Without the right retrieval cues, information can be stored in long-term memory, but never be accessed. Most researchers agree that information stored in LTM is there permanently--all it takes to access that information is the right cues.
Sensory Memory is essentially the first stage in the memory process; it is the point where information enters the nervous system through various sensory systems (sight, taste, sound, etc.). Information that has been encoded into sensory memory takes the form of neural messages that travel through the nervous system. The memory is accessible as long as the neural message is still moving through the nervous system to be further processed in the brain.