The study of criminology is a broad academic field. One textbook that I have (Criminology, by Larry Siegel, 11th Edition) briefly defines criminology in two related ways. It says that that criminology is either an “integrated approach” or a “scientific approach” to the study of “criminal behavior.” This means that criminology is a very broad field, bringing in aspects of such fields as sociology, political science, psychology, policing, and biology (among others).
Criminologists need to study sociology because they believe that crime is influenced in part by social factors. They believe that people are influenced by the environment in which they grow up and by the kinds of relationships they have with various other people. To understand crime, then, criminologists need to understand how society impacts the behaviors of individual people. Of course, society is not the only influence. There are biological factors that appear to influence crime and criminologists need to try to understand those as well.
Criminologists also have to understand the process of making laws. Actions are only crimes if the law defines them as such. It is therefore important to understand why some actions are considered criminal and others are not. It is also important to understand why laws against some criminal actions are stronger than laws against others. (A good example of this is the different treatment of cocaine and crack cocaine in American laws.)
Criminologists ultimately seek to understand crime in order to minimize and control it. They therefore try to determine what programs and methods can be used to prevent people from committing crimes. They study the efficacy of various types of policing as well as preventative programs like those meant to prevent crime by keeping kids “off the street.”
All of this is done in a scientific way (as far as is possible when dealing with people and society). Criminologists are supposed to use scientific research methods so that their conclusions will be valid. In short, then, criminology is the scientific study of why people commit crimes and how crime can be controlled or minimized.