Chinese team TaiG has released an iOS 8.3 untethered jailbreak tool on Windows for compatible iPhone, iPad and iPod touch devices. TaiG warns that some jailbreak tweaks are not compatible with iOS 8.3 as Jay Freeman (saurik) has yet to update Cydia Substrate/Mobile Substrate to be compatible with the latest jailbreak. An iOS 8.3 jailbreak tool for OS X has not been released.
Jailbreaking enables iPhone, iPad and iPod touch users to install unapproved third-party apps, tweaks, mods, custom themes, utilities, and other content (called "packages") from Cydia, which is essentially an App Store alternative for jailbroken devices. In other words, jailbreaking allows users to further customize and enhance their iOS devices beyond the limits that Apple imposes.
It is highly recommended that you back up your data through iTunes or iCloud before attempting to jailbreak your device, as there are various issues that could require downgrading or restoring your device and result in data loss. TaiG offers more information on its Twitter and has posted step-by-step instructions on how to use the iOS 8.3 untethered jailbreak on its website.
TaiG 2.0 supports iOS 8.1.3 - iOS 8.3.