You don't want to mess with share permissions too much. They are rather basic and only apply when a user is accessing a file or folder over the network. NTFS permissions apply to all users, local or network.

When determining what applies to whom remember this: when combining share and NTFS permissions, the most restrictive permissions between the two will be applied. For example, on a folder you have share permissions: everyone, read and NTFS permissions: everyone modify, it will take the most restrictive permissions, so someone accessing this folder across the network would only have read permissions. Lets the same person logged on to the machine locally, share permissions would not apply, because he is not going across the network. In this case only NTFS permissions would apply so he would have modify permissions.

Since the most restrictive permissions are used, and NTFS permissions apply to everyone while share permissions only apply to network users, it is common that an admin will set share permissions to full control - everyone. The admin will then use the NTFS permissions to restrict the files and folders as necassary. By putting share permissions at full control - everyone, the NTFS permissions will always be the most restrictive. This simplifies where ot set the permissions, while giving you more control as NTFS gives you more options for restricting access.