The update patches 5 vulnerabilities in Safari (2), WebCore (1) and Webkit (2).
The update can only be installed through iTunes.
Nathan Kline confirms reports that the update reverses any hacks. If it detects any alteration, you are invited to restore your iPhone.
Nathan did note that IMAP email has been improved allowing one to see their folders on server, even the ones created by the user. This allows the same functionality that a user would have on their computer’s email