Read 7 Things You Need to Know About Ning.
The bottom line is that a Nings is a social networking tool that allows you to collaborate with people in your field of interest from all over the world. There are many different things you can do in a Ning, and there are many different Nings out there that cater to specific interests. So go out and explore these online "group gatherings" to see what they are like and the advantages of belonging to one.
Here are three Nings to try looking at:
Future of Education Ning
Ning for Teachers
Teacher Librarian Ning
Texas School Librarian Ning
Discovery: Go to either one of the Nings above or find one that intrests you and look at the different features and options then write a blog post about what you think a Ning would be good for and how you might use it.