Difference between semi structured, unstructured and techniques in research

Structured interviews are such that the interviewer comes prepared with a list of questions he wants to ask and the respondent simply answers what he/she is asked. They are useful for researches that can easily be coded and in most cases, the codes are already on ground. You basically just fit the responses in. They are close-ended

For semi-structured, the questions are open ended and the respondent is allowed to talk/respond in details. You come into the interview with an interview guide which may change as you proceed. For instance, the respondent may say something you have not considered but you now find useful. You can include this new ideas in your next interview. They are particularly helpful with inductive methodologies such as grounded theories.

The way I view unstructured interviews is 'listening to a gist'. You simply listen to the respondent talk about his/her experience. It is more story telling and you pick what you want from the story, hence the name 'unstructured'. You are however allowed to seek clarifications. The best way to this form of interview is to build some trust and 'closeness' with your respondent. Apparently, such approach will be useful in phenomenological studies.

Finally, the unstructured and semi-structured interviews allow respondents to express themselves freely and can be rewarding, even though time consuming.

I hope this helps.