Interviews are basically of many types. However, you can broadly divide all kind of interview in two categories:
I will tell you about each of them.
Structured interviews are those in which questions are pre- defined. The person who takes the interview have a list questions and they ca ask only from them. Technical interview are one of the example of structured interview. You must have gone through some kind of technical interviews. What is your idea about those interviews? You must have found that same questions were asked to all of the contestants. You can take an example. Suppose you have to give an interview of java professional. Then it's well known in advance that you will be asked question about java. I must tell you that there are some pet questions in java which are more often asked. You must also know this.
These are the types of interview which are different in almost the other same interview which you will face. However I must tell you that these kinds of interviews are also very important. Various surveys which are conducted via telephone are an example of unstructured interview. HR interviews are also example of unstructured interview.
You have just learnt about structured and unstructured interviews. I must tell you that as far as structured interview are concerned, you can prepare in advance. But the mystery of unstructured interview ends only on the date on which the interview is to take place. Hence you can say that the structured interviews are easier than unstructured interviews. But this is not really the case. There is always a chance that you can predict the questions. Interviews like screening test can easily be understood and you can easily tell what questions will be asked. Most of the interviews are verbal and you will hardly find any sample because no body gets any paper. However many groups put forward some questions which they remember. You can really go through them and learn what is required.
However, you will find out that the questions of non structured interviews are so general that you can hardly need to study in advance. They might ask from you about your hobby or your favorite game. Now you only tell that what preparation you can make to answer to these questions. Really you are required to make no effort and hence you should really not worry. However the structured interviews needs some study. If you are giving technical interview on java, then you will have to study the details of the java language. Only then you will be able to pass these interviews. I must say that unstructured interviews require smartness whereas the structured interview requires study.