when setting permissions through the security tab you are setting
permissions in the filesystem (ie. on files and folders).
Those permissions will *always* apply, no matter.
when setting permissions through the sharing tab you set permissions that
only apply when user access the folder over the network, like the
description says.) So those aren't always applicable, only when coming to
the folder (ie. the shared definition of the folder) over the network.
hope this clear it up for you.
george