Serena, that is a great question. Criminal Investigators is a title given to anyone investigating crime or evidence. A Detective is a title obtained by a law enforcement officer who has worked at either a police officer or deputy and passed a test (Detective exam). A detective has to have a required amount of years with the law enforcement agency before being eligible to take the exam, this person also must have a certain level of education and some required training hours before taking the exam. Once a detective title is obtained, that officer/deputy, works on investigating crimes more thoroughly than at the level of police officer or deputy. They question victims, suspects and other involved people in an incident to gather information and evidence to solve the case. Detectives assigned to Civil Offenses usually deal with family issues or issues between people where wrongs have occurred, like contracts, evictions, warrants, protective orders and the like. Detectives who work criminal cases work either property crimes (burglaries, criminal mischief, recovered property (stolen goods) and the like). These detectives also might be involved in the narcotics sections, going undercover, surveillance and serving warrants. Other detectives work major crimes, which include homicides, sexual assault, assaults, robberies and officer involved shootings. These detectives tend to always be on call, which means they may be called out at any time day or night, weekends or holidays.
Criminal Investigator is a title that can be given to a civilian employee (local, state or federal) who assists in investigating crimes or situations, but who are not law enforcement officers. However, some agencies do use that title in law enforcement employment positions where the officer or deputy has not taken a detective exam. Typically, District Attorney or County Attorney offices use retired law enforcement officers as their Criminal Investigators to help investigate cases going to trial. These men and women are not lawyers, but help gather information or people to provide witness testimony in cases going to trial.
I hope this helps you understand the differences in these areas of investigation. While both Criminal Investigators and Detectives investigate cases, crimes, and look for information. Only the Detective can collect evidence and has the authority to examine a person - interrogate, interview, or process an involved person for evidence.