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".)