Normal and abnormal psychology essentially share only one common characteristics. Both normal and abnormal psychology study behavior and mental processes. Outside of this, normal and abnormal psychology are very different. Essentially, the boundary of normal psychology is where abnormal psychology begins.
Normal psychology studies the range of behaviors and mental processes which show no signs of abnormal mental processes or behaviors. Outside of this range, when behaviors and mental process begin to show abnormalities, is where abnormal psychology takes over.
Normal behaviors do not interfere with the functioning of an individual or infringe on others. Abnormal behaviors, on the other hand, do affect the individual's mental and behavioral processes (which infringe on others). Essentially, abnormal psychology is the study of mental disorders.