Google + is Google's answer to Facebook.  At its core is the idea that people wish to connect in many ways through one profile but also want to insure that what they share goes to a limited audience or "circle" that they choose.  In our daily, face-to-face lives, we share information with our spouse that we'd not share with our children.  Our "circle" with our spouse would be a single person.  Our "circle" with our children would be one, two, three, or more.  Our "circle" with our family could be in the tens to 100's depending on the size of your family.  The same is true for school.  There are different audiences that teachers would want to share to.  For example, our parent community.  We might want to share general homework reminders to them and more specific reminders to our students.  Each could be in different circles and receive the same or similar information.