Short term memory is the ability to keep information in mind for a short amount of time. Remembering a phone number long enough to find a piece of paper is an example. The brain keeps information in its short term memory for a small period of time. The amount of time the mind is able to do this is usually some seconds.
The opposite of short-term memory is long-term memory, which is where things are stored in the brain for a long time. These items may be recalled later, or there may be difficulty recalling them.

Memory 
Memory is what goes on in the brain in storing and retrieving past experiences. These processes do what we call remembering. Memory is the foundation of  intelligence and the ability to learn. 

Information about a needle being sharp and hurtful when touched by bare hands is processed to make a response of survival and self-preservation. This leads to us (and other animals) to change what we do when we met needles a second time. Memory, which starts out as short-term memory allows for learning, and helps intelligent behavior.

Basis of memory 
Donald Hebb (19041985) was the first to come up with a way to tell the difference between short-term and long-term memory. When the brain receives an input, like something that is seen or heard, a sensory memory keeps a copy of what is seen or heard, but this memory lasts for only about a few seconds. Anything that is kept in the brain longer depends on our selective attention. Things that we "notice" are kept in short-term memory for up to a few minutes. This memory is thought to be dependent on electrical activity in the brain and is easily destroyed by interruption or interference.  Short-term memory includes iconic memory as well, which holds visual images; acoustic memory, to hold sounds; and working memory, which is an active process to keep a memory until it is put to use.

Memories which are stored for longer than this are called long-term memory. Hebb had a theory of long-term memory as  a change in the strength of connections between the neurons. Long-term memory is relatively permanent storage and the process involves the creation of new proteins. 

Whether or not the information is stored in long-term memory depends on its importance. Usually for animals memories that are made during times of stress and discomfort are important for adaptive values. In the future they help avoid anything that will bring them stress.

Duration 
Short-term memory has a limited duration, and the contents decay over time. The decay idea is usually paired by the idea of repeating the memory over and over. In order to overcome the limitation of short-term memory, and retain information for a longer amount of time, it has to be repeated either by saying it out loud of by saying it mentally repeatedly.  By doing this the information will re-enter the short-term process and be kept for a further period.

Capacity 
Forgetting greatly limits the information that can be kept over a short period of time. The capacity of short-term memory is finite, but there is no clear unit of measurement for what that is. Memory span is the term used to describe this, where there is only a certain amount of information that can be memorized over a short period until it is lost.

Factors affecting STM 
The capacity of short-term memory leads to the question of the capacity of short-term memory as a whole. At present there is no way of telling whether a piece of information will be stored as short-term memory or not. In an experiment done to determine the capacity of short-term memory individuals had to memory a list of items and recall it, and it was suggested that the span of their short-term memory was reached when they could no longer remember the list in order.  

There are many conditions that can affect short-term memory. They range from simple distractions to serious disorders. The loss of memory is natural and is expected with ageing. By the age of 55 it is found experimentally that the loss of memory starts to occur which usually occurs more often with long-term memory loss. Age very much affects the short-term memory as well. 

There are many other things which affect memory loss beside advanced age. There are disorders such as Alzheimer’s disease, aphasia, schizophrenia, advanced age, and posttraumatic stress disorder. These are all linked to the loss of memory.

References 

Memory
Brain