Sharing allows users to give other users or groups access to specific information. A good practice is sharing only the necessary objects among the smallest set of users possible.
The owner of an object can choose to mark the object as Private. An object that is marked as Private can be accessed only by the user who created the object and by those who share the object (for example other Users added to this User's private Group).
For objects that support sharing you can right click and select from the context menu the "Sharing" command (Ctrl+R)
Private: object can be accessed only by the owner or users in the private group of the owner
Public Read/Write Access: everyone can access and modify the object
Advanced: Custom sharing - you can choose what users or groups will have access and edit rights
By checking the "Save as default for the current Entity type" the application will use the currently selected access control settings whenever a new object of this type is created.