Parents' meeting is only between parents, there is no involvement of teachers. A parent-teacher meeting is a social gathering of parents and teachers. A parent-teacher conference is a special case of meeting. Here, discussions and presentations also take place. There is lot of knowledge sharing.
Sorry, Neeraj, I have to disagree with you. These all mean the meetings between a child's parents and schoolteacher.
I suppose a parents' meeting could be just between parents - but it is also used for parent/teacher meetings. Parent-teacher meeting is not a social event (why would they want to socialise with each other anyway?)
In fact, I want to distinguish the following two types of meeting:
(1) all parents of students in a class sitting in a classroom, listening to a head teacher speaking about their kids every semester. (not one to one, and the teacher only speak their study generally, address common problems.
It probably varies from country to country and school to school - it's not as though these uses are set down in law somewhere.
Over in the UK
Parents meeting - could be one-to-one with teacher, or could be a group of parents meeting without any teachers, or could be the situation you describe with a group of parents being spoken to by the school.
In my country parent teachers meeting is one person chairing the business aspects of a meeting with many parents discussing subject matters that affect their children and the school in general. Example: Planning a fund raiser, or say a building repairs project.
While parent teachers conference would be one teacher and parent in a one-on-one discussion about one child.