Unlike Facebook or Twitter where you can only to communicate to members of the respective networks, Google+ allows you to send updates to anyone with an email address. As well as posting to individuals or Circles you can message a person at their email address.
Recipients of such emails can read the message they are sent and are also invited to join Google+. You can post to Circles and to email addresses simultaneously; for example, you could send a message to a Prospects Circle together with one or more email addresses.
You can also add people who aren’t on Google+ to Circles by adding their email addresses. When you post an email address to a Circle you’ll be asked if you want to email your update in the same way as if you’d shared directly to their email address.