Informational interviewing is a method of conducting research in a particular career field by talking to people in that field. It is a tool for gathering information. You can learn more about what it is like to work in a particular field, what skills and credentials are required and what the career path might be like. People like to talk about their careers and to help you with yours. The informational interview is not, and should not be, used to ask for a job.
Directed interviews are the most common type of interview that employers conduct. They will want to explore your background and experiences and find out if your skills and knowledge dove tail with their needs. In a directed interview the employer will ask you to relate your skills and experiences to their business situation. You can familiarize yourself with this format in advance and be prepared to address their questions.
Panel interviews, in which candidates meet with multiple interviewers at the same time, are a common interview format in academia and public sector organizations. Its many benefits are now making it a more popular structure for corporations and private sector businesses as well. Panel interviews can be intimidating, but with proper preparation you can be successful.