What is the difference between share level security and user level security?

User level file permissions pertain to users that are located on the domain/local computer.. Share level permissions apply when you are sharing out a file/folder.

When you share out a file/folder you need to make sure that you allow the user/group permission in both the Share level permissions and the user level permissions.

If you have a file located on a local computer that another local user may need access to then you would only require user permissions to gain access to the file.