Sharing is allowing specific users to have access to your web site to maintain content.

How to Share Content
One of the great features of the web system is the ability to share your site with others for the purpose of adding and editing content. You can share as little or as much control as you wish over a page, folder or the entire site.
Permissions specify exactly what a user can do. Permissions specify exactly what a user can do. They're additive, meaning that "can view", "can add", "can edit", "can review" (i.e. review and publish), and "can manage" are actually individual permissions that when, enabled together, allow a user to function in their given "role" on a site.
Review List: Using the Review Process
When several people contribute to a site, the site manager (or anyone with the 'Can Review' permission) may choose to utilize the review process workflow to maintain control of site content.