In psychology, memory is the process in which information is encoded, stored, and retrieved. Encoding allows information from the outside world to be sensed in the form of chemical and physical stimuli. In the first stage the information must be changed so that it may be put into the encoding process. Storage is the second memory stage or process. This entails that information is maintained over short periods of time. Finally the third process is the retrieval of information that has been stored. Such information must be located and returned to the consciousness. Some retrieval attempts may be effortless due to the type of information, and other attempts to remember stored information may be more demanding for various reasons. 

Short-term memory is believed to rely mostly on an acoustic code for storing information, and to a lesser extent a visual code. Conrad (1964) found that test subjects had more difficulty recalling collections of letters that were acoustically similar (e.g. E, P, D). Confusion with recalling acoustically similar letters rather than visually similar letters implies that the letters were encoded acoustically. Conrad's (1964) study, however, deals with the encoding of written text; thus, while memory of written language may rely on acoustic components, generalisations to all forms of memory cannot be made.
Which kind can it be used for?
Short-term memory is believed to rely mostly on an acoustic code for storing information, and to a lesser extent a visual code. Conrad 