When you post on Google+, you can share things you like, or start a conversation with other people.
When you share, you’ll choose who can see your post. For example, you might want to share an article with everyone in your circles, but share a photo of yourself with only your Family circle.
When you share something using Google+, you can share it publicly, with your circles, with individual Google+ users, or with email addresses. And of course into Communities that you are a member of.
If you share with others using an email address, they'll get an email that includes either the content you shared, or a link to your content (unless they've opted out of getting such emails).
If they share the email itself or the link with others, people you didn't directly share with may be able to see your content.
Some features on Google+ require that all users have a Google+ profile, for example the Communities membership.