Interviewing is the simple act of asking questions of someone to gain information. It is a critical part of almost any investigation. At its most basic level it can be done by anyone. But interviewing is a skill and often terrible interviewers will believe they are good at what they do.
What complicates interviewing is that many of the people being interviewed either do not realize they have important information or do not wish to give you the information they do have. What further complicates the interview process is that our personalities and actions will directly affect the information obtained in every interview.