An interview is simply a tool used by a potential employer to assess a candidate's ability to perform a role. The interview will normally be the first time that the employer has the opportunity to meet you. They need to assess whether or not you have the qualities to perform the role competently, the experience that you have so far in a similar role, and also whether they like you as a person and whether you are likely to fit into the team environment. It is also your chance to find out if you want to work for the company, so it is a two-way process.
There are different kinds of interviews though, and some jobs will require more than one, and there may be more than one interview, so it's just as well to be prepared for this eventuality.
Face-to-face interviews - This is the kind of interview we expect when invited to an interview. It may be one-to-one between you and the interviewer, or you may sometimes find that there are two interviewers, such as a functional specialist and a member of the resourcing or HR team. Don't assume the interviewer has read your CV. Go through it with them, don't go into excessive details, but do engage them in conversation.
Panel interviews - Panel interviews, where several people interview you at once are common for positions of importance, and it is particularly popular in the public sector. Recognise the different roles that may be adopted by each member of the panel. Usually there will be a chairperson to coordinate the questions and it is quite common for at least one panel member to adopt a "bad cop" role - where their questions may seem abrupt, even rude. This is a deliberate tactic to see how you react. Try and stay calm, don't let them rile you. If they talk over you, stop and let them ask their next question. If you know it is a deliberate tactic, you can behave appropriately.
Telephone interviews - Telephone interviews are increasingly used by companies as an integral part of the recruitment process, often at the early stage of selection. If you are offered a telephone interview, the most important fact to remember is that the employer wants to find out the same information as they would face-to-face, so your preparation needs to be just as thorough. These interviews are usually carried out by someone in HR person who may know very little about the position and technical terms related to a specific job. Make sure to use clear vocabulary, and avoid buzzwords and jargon while answering their questions, but be descriptive; you are trying to paint a picture with words, since the interviewer has no visual clues of you.
Group interviews - Several candidates are present and will be asked questions in turn. A group discussion may be encouraged and you may be invited to put questions to the other candidates. Sometimes there will be role plays or group tasks, where you will actually have to co-operate with people who want the same job as you. This is particularly common for jobs that involve team working skills. Group interviews are designed to see how you interact with other people so be courteous to your fellow candidates.
Sequential interviews - Here you may face several interviews in turn, with a different interviewer each time. Usually, each interviewer asks questions to test different sets of skills. However, you may find yourself answering the same questions over and over. If this does happen, make sure you answer each one as fully as the time before and be consistent.
When you are invited to an interview it is perfectly okay to ask what form it will take, that way you will be more prepared for whatever they throw at you.