Humans have basically three forms of memory: Sensory, long-term and short-term. Long-term memory is just like hard-drive space. Similarly, short-term memory functions like a very small RAM. This short-term memory is capable to hold only about five to nine (seven is an average) items at a time.
Retrieving information longer than this will need you to either pack it together into seven units or store it in long-term memory.-- Mind Cafe