The main job of a police detective is generally to conduct criminal investigations. This can involve many different things depending on the kind of crime being investigated. For example, sometimes there is a lot of computer work involved in being a police detective, and in other cases, the job can be almost entirely devoted to interviewing people. Fundamentally, detectives are expected to piece together all the information about the crime to discover who is responsible and provide evidence that can prove a suspect's guilt in court. In some police departments, detectives might be assigned to different divisions devoted to specific types of crime, while others have more general-purpose job descriptions.
The job of a police detective is often a fundamentally different job than a patrol officer's, for a lot of reasons. Patrol officers are usually called in either before, during, or immediately after a criminal act, and they're expected to handle the immediate work of protecting the public and the initial capture of any criminals. Police detectives are primarily devoted to solving crimes and are usually called in after-the-fact. They're still devoted to protecting the public, but their purpose is generally to prevent a suspect from re-offending by discovering a suspect's identity or making sure the guilty individual is punished by the court system by helping find evidence to prove the case.
The nature of the job can depend a lot on the types of crimes a detective specializes in and the detective's level in the department hierarchy. For example, a homicide detective's investigations will sometimes involve totally different elements than a detective focused on narcotics, and when it comes to hierarchy, lower-level detectives might be assigned to basic information-gathering in a major investigation with a whole team to help in the task, while a more experienced detective heads up the investigation.
A police detective is usually deeply concerned about the courtroom implications of his actions. He will generally have to testify, and if he makes a mistake that leads people to question his credibility, a case might be ruined, which could potentially endanger the public. Detectives often have to take special precautions and even do things they might consider redundant to ensure that every reasonable lead is followed so that the case looks as credible as possible. Even if they're almost certain about the identity of the guilty party, they may conduct additional interviews and perform further investigations to rule out every conceivable possibility. In order to follow this kind of strict methodology, detectives often have to perform exhaustive, repetitious actions, and most detectives suggest that the job isn't as romantic or exciting as it might appear in many movie portrayals.