Cognition is a term referring to the mental processes involved in gaining knowledge and comprehension. It includes both conscious and unconscious processes and encompasses everything from perception and recognition to higher-level processes.
Cognitive psychology is a sub-field of psychology that investigates how we acquire knowledge and process information. It includes the studying of our mental processes on attention, learning, language, perception, thinking, memory, reasoning, creativity, problem-solving, judgement, decision-making.
The field of cognitive psychology can be applied in many different ways. A psychologist might try to help you improve your memory by studying how you process information or react to different types of stimuli. They might also try to help you organize your thoughts by studying how you think about different topics and relate them to one another.