The way job interviews are undertaken can vary widely. Some facilities will ask you to take an online test first before your interview. This may be to see if you have a similar philosophy to that of the organisation. Some will have a single face to face interview with a panel or 3 - 5 people which usually will take about 20 minutes. Others may ask you to participate in a group exercise where a problem is to be solved. This is to see how well you perform in a team, how well you communicate and how well you can think critically in a challenging situation. So your interview may take anything from 20 minutes to 4 hours, may involve a small or large number of people and may include some extra online work.
Other job interviews are by phone or by video social media sources such as skype or hangout. In general the employers are trying to find out if you will be the best 'fit' for their organisation, while you are trying to find out if this is the place where you want to spend at least the next 12months of your life.