Places is the name of the overall concept. A Place can be a Space or you Personal Content Area, where you publish your personal blogs or documents, or where you start a private discussion. There are also Social Groups, which also fall into the Places concept, but we don't use them here (yet). And then there are Projects, which are also Places but from my point of view they don't make much sense in our context, so better forget them.
What you do in Places is creating Content. THen you can follow that Place and get notified.