A button on a website or on the Facebook site that allows users to share Pages or other content with their own network. Shared content appears in the Newsfeed of the user who chose to share it, where their Friends can see it. The Shared content links back to the original Page or website from which it was Shared.
Why you need to know about the Share button:
Marketers can add Share buttons to website or blog content, to increase social sharing among readers. This allows existing readers to expose their social network to the content. Facebook users can also Share content posted on the social network by a Facebook Page or personal profile by simply clicking the Share button, as shown below.