No one can say for sure, but usually these interviews are quite casual compared to a job interview. The main goal is for them to get to know you and basically interact with you as a person instead of just a pile of papers from your application package. They will also want to tell you about their program. This is unlikely to be an "exam" type interview (but I can't say for certain of course!).
Here's what you should know / how you should prepare:
1. If it's a Skype or phone interview, ensure that you have a quiet place to conduct your interview. Your own home is best, ensure that others are not around or that they know you have an interview. If your school allows you to book rooms (some schools have rooms specifically for this), then do that. Ensure you have a good internet connection. For Skype, also make sure that whatever the camera view is, it is something appropriate for an interview (blank wall is best). Test out the connection and check the camera view if necessary.
2. For every item on your resume/CV or in your essay, be prepared to speak about them. Don't assume the interviewer has read and have remembered all the details in your application package. Practice a 30-60 second summary of everything you write about in your resume/essays so if they ask something like "Tell me more about X", you can go ahead and do that. Be knowledgeable about these topics, especially the research experience (if any), in case they ask follow up questions. Be prepared to speak about your specific contributions to each project.
3. Match your tone to the interviewers. They will likely start casual and maybe even make some small talk at the beginning. Be prepared for this. I know interviews can be stressful, and sometimes people go into them with a "battle" or "exam" mentality, while the interviewers are looking for a friendly chat. This can make it a little awkward! So, react based on their cues and remember that it's more likely than not that they are looking for a friendly interaction.
4. Be prepared to answer a question like, "Tell us a little more about yourself" and other questions about your career goals, what other programs you are applying to, and what you are looking for in a graduate program. Feel free to repeat stuff from your essays. I mention this because sometimes people only prepare for the "science" part of the interview and then struggle when switching to "personal" topics. Although I say these are "personal", also remember to keep it professional. Remember that scientists are people too, so it's okay to talk a little bit about your hobbies. That said, I would weigh most of my answer on research related goals and topics.
5. Finally, make sure you have 1 or 2 good questions to ask them about their program. You probably have some questions on things like what classes are like, what quals are like, whether you are guaranteed summer funding etc. so it's fine to ask them. It's also okay to ask questions that come up based on the conversation. If you know who the interviewer(s) are, and if they are doing work directly related to your interest, you can also ask a question about their research. However, I would say for a short 15 minute interview, you can just ask a simple logistics type question.
good luck! Don't be surprised if you actually only speak for a few minutes and they spend most of the time talking to you instead