The most critical patch included in the package is an update that fixes a bug in iOS that allowed anyone to bypass the passcode security to grant them permission to access the phone dialer alongwith another patch for a vulnerability that allowed a potential attacker to have access to voicemail and photos.
The update is available for iPhone 3GS, 4 and 4S, iPhone 5, all iPads following iPad 2 and all iPod touch devices after the 4th generation iPod touch. As usual the update is being rolled out in waves so one can expect to see a notification sooner or later.
However the not so exciting news is that the update also patched several holes that Evasi0n exploited to perform an untethered jailbreak on all iOS devices, including the latest iPhone, iPad, and iPod Touch. People who upgrade to iOS 6.1.3 will no longer be able to use Evasi0n to jailbreak their devices. And once on 6.1.3, newer Apple devices cannot be downgraded to a previous iOS version.