If you upgrade iOS with iTunes, your device will be un-jailbroken and Cydia packages will be deleted. Before upgrading iOS, make sure a jailbreak is available for the new version.

As of October 29, 2014

Untethered jailbreak for 8.0-8.1

Pangu Team has released an untethered jailbreak for iOS 8.0-8.1.

Currently the website (http://pangu.io/) and jailbreaking tool are only available in Chinese, and the tool is only for Windows. The jailbreaking tool does not yet install Cydia automatically.

If you're not comfortable experimenting with a potentially confusing or buggy jailbreaking experience, you may want to wait for news from @PanguTeam and @saurik before jailbreaking.

How do I find my current version?

Open the Settings app, tap General, tap About, and look at Version.

(Your device's version is also listed at the bottom of the Cydia "homepage".)