What’s the difference between a ‘formal’ and ‘informal’ workplace investigation?

Q: “I am taking investigations training and they make the distinction between a formal and informal investigation. I am confused as to what the difference is, since both in my opinion require the same process with regard to talking to people to find out information. I am obviously aware that investigations in general are required by law; for example, in allegations of harassment. Can you please explain the difference?” – Lara, New York