Is an invitation to 'chat' about a potential PhD position actually an interview? What am I to expect?
Yes, this is an interview. Or at least a part of an interview - depending on your location, there may be a more formal one later. As a rule of thumb, both for academia and industry, whenever you meet with a prospective employer, be it for a coffee or lunch, a &quot;chat&quot; or something else, it is part of the interview.
As for the chat itself. If he asked you to give a short presentation, you should prepare to give a rather thorough one; ie. be prepared to explain basic stuff as well. If you have no idea about the format (time, place, equipment, audience (eg. is it just him?)), ask him. In my field it would be common to prepare some slides for such a presentation, but for others you would do without. It is always nice to know roughly how long time you have when preparing. Important: If the prof. says that you have, say, 20 minutes, then prepare to stick to that.
You can ask about details of the research, what is expected of PhD students wrt. teaching, possibilities for traveling and other things you are curious about.