The difference is that
1. will set permissions for users to be able to access the shared folder, if a user is added here they will not be able to even access the folder.
2. This is set for NTFS permissions
This is so that there is tighter security in Microsoft folders.
Hope this helps
Those are both true statements, I would also Like to add that the way you give permissions to share and Security Tabs will effect the way users can access files/folders in them. In Server 2000 and 2003 There is the Effective Permissions tool, which will help you to determine what will happen when you check the different persmissions. This is under the Advanced button on the Security Tab. Remember that Windows usually take the most restrictive of permissions when combining. Keep that in mind when setting permissions.