Folder Shares: question about security

Hello,

When I share a folder, I see two tabs in the properties window: Sharing and
Security. In the Sharing tab, there's the Permissions button that allows me
to add users to access this shared folder, and then in the Security tab it
allows me to do the same with more granular options.

What is the difference between these two security models? If I were to
simply grant a user in the same domain to access a shared folder, which of
these two options should I use. On a side note, what if the user belongs to
another (but trusted) domain, would it still be possible for that user to
access the share?

Thank you.
.