Psychology is a scientific study of  human behavior and mental processes. It covers everything that people think, feel and do. Psychologists differ in how much importance they place on a specific type of behavior, some believers that you should study only that you can see, observe or measure directly while other psychologists believe that fantasies, thoughts and feelings are also important even though these behaviors are not directly observable. Consider the story of the blind men and the elephant. A long time ago, three very wise but blind men were out on a journey when they came across a sleeping elephant. Because they could not see the elephant,they did not know  what was blocking their way so they set about to discover it. As it happened, each man put his hands on the different parts of the elephant the first man, having felt the elephant's trunk, describe a creature that was long, wormlike, and quite flexible."No, no! You must be mistaken," this creature is wide, very round, and does not move very much." The man who was holding one of elephant's tusks added his description of a small, hard pointed creature. Each of these men was correct in he's description  of what he felt, but in order, to understand the elephant fully, they needed to combined their accumulated knowledge. The study of human behavior is similar. Many different approaches are necessary to understand the complex richness of human behavior.
 Sensation and perception is one of the interesting topics in psychology. It tells how your senses works and how your brain interpret things that surrounds you. To better understand sensation and perception let's define it first.
Sensation is the passive process of bringing information from the outside into the body and to the brain while perception is the active process of selecting, organizing and interpreting information brought to the brain by the senses.
When you hear your mom shouting, soundwaves enter the pinna travel down the auditory canal, vibrate the eardrum, set the ossods in motion, move the hair cells, which change the vibrations into neural impulses. This is what you called sensation, what about perception? Perception takes place when you are able to recognize that your mom is shouting. this is only an example of how sensation and perception help us to recognize and interpret things in our environment. 
Perception always entails sensation as a part of itself. Sensation in return, never happens in adult life without perception hearing these. This two processes always help us to recognize and appreciate the things that is present in our environment.


Reference: Understanding Psychology : fourth edition