Here are examples of what this looks like: on iPad, on iPhone. This is caused by having a tweak incompatible with iOS 6 installed, and it only shows up after rebooting. Check your tweaks and uninstall any that might not be compatible - this list may help. To uninstall incompatible tweaks: first put your device into Safe Mode using SBSettings (tap the "Power" button in SBSettings for the Safe Mode option) or using a similar tweak, or if you don't have something like that installed, you can reboot your device while holding the volume up button to disable all tweaks. Then open up Cydia and uninstall any tweaks that you suspect are incompatible. You can reboot again to check, and repeat the process as necessary.
Known tweaks that cause this issue are the current versions of Deck and Binary Clock, and old versions of SubtleLock, Incarcerapp, NowListening, Transparency, and RetinaPad (please update them if you have them installed). Respringing is a temporary fix for this issue (it will make Cydia usable again), but unless you remove the incompatible tweak(s), you will get this problem again the next time you reboot your device. Winterboard and UIKit Tools do not cause this problem (they are fully compatible with iOS 6), but installing/updating them may cause you to reboot your device, which makes the incompatible tweak problem show up.
Why is the Weather app crashing?
This may happen if you have AppSync installed, due to bugs in AppSync. You can usually fix this by uninstalling AppSync and rebooting your device.
Why am I getting this weird "Wow" error message in Cydia?
If you get the "Wow, you exceeded the number of package names this APT is capable of" error, that means you have more than 65,535 packages available from your installed sources, and Cydia cannot handle this many. Please go to Manage -> Sources on iPhone and iPod touch (or Sources on iPad), tap "Edit" at top right, and remove several sources. It's best to keep the default repositories (BigBoss, Cydia/Telesphoreo, Dev Team, ModMyi and ZodTTD/MacCiti).
Why isn't Cydia displaying my SHSH blob versions at the top of the Cydia homepage?
Cydia is under such heavy load that saurik has to limit this feature for now. You can save them yourself by using TinyUmbrella - download it, install it, plug your device into your computer, and click the "Save SHSH" button. (See here for some background information from about the SHSH servers, if you're curious.)
Why aren't notifications showing up on my lockscreen with this HTML theme?
Winterboard has a bug preventing lockscreen notifications from being visible when a lockscreen HTML theme is enabled. saurik is aware of this issue and will look into it as soon as he can.
How should I prepare my device for jailbreaking with evasi0n?
If your device is currently on iOS 6.0, 6.0.1, 6.0.2, 6.1, or 6.1.1, make sure to back up your device with iTunes or iCloud before jailbreaking, temporarily disable your passcode (if you have one enabled), and that's all the preparation required. If your device is on an older iOS version, or if you want to be extra sure that the jailbreak will go smoothly, you can follow these steps:
Make a backup with iTunes (click "Back Up Now" in iTunes).
If your device is jailbroken, write down a list of your installed packages, because the next step (restoring) will remove the existing jailbreak. If not jailbroken, ignore this step.
Restore the device with iTunes, which will update your device to the latest iOS version, currently iOS 6.1 (or iOS 6.1.1 for iPhone 4S). (If you have an iPhone 4 or 3GS with an unofficial carrier unlock that depends on your device's baseband version, restore your device with custom firmware instead.)
Select "Set up as a new device" on the device itself during the setup process after the device is restored. Do not select "Set up as a new device" or "Restore from this backup" on iTunes yet.
Open evasi0n on your computer and jailbreak your device.
Open up Cydia for the first time to allow it to "prepare the filesystem". Wait for the respring to happen.
Start iTunes again and select "Restore from this backup" to put your data back (apps, music, etc.).
You're done. Feel free to open Cydia and install some packages. To access previously-purchased packages, tap "Manage Account" on the Cydia homepage and log in.
What is a jailbreak?
Jailbreaking your device means installing a small program that removes restrictions in the default software. A jailbroken device can run apps and extensions (themes and tweaks) not approved by Apple. Jailbreaking does not slow down your device or use extra battery, and you can still use all your existing apps and buy new ones from the App Store. Jailbreaking simply enables you to do more with your device; nothing is taken away.
Will evasi0n unlock my device? Does my device need to be activated before jailbreaking?
No, evasi0n does not provide a carrier unlock. (And if you're curious about the legality of unofficial carrier unlocks, see these explanations.) Yes, your device needs to be activated before using evasi0n.
What if I decide that I don't want the jailbreak anymore?
If you someday decide that you want to undo your jailbreak, you can plug your device into your computer, make a full backup with iTunes, click "Restore" in iTunes to wipe the device, and load your backup when prompted. All your App Store apps and the information in them will be preserved as usual.
Can jailbreaking "brick" my device?
Jailbreaking cannot put your device into an unusable state on its own. You will have full access to your jailbroken device, which gives you the power to modify it in ways that can put it in a state where you have to connect your device to iTunes and "restore" from a recently-synced backup. However, it should not be possible to render your device as permanently non-interactive as a brick, no matter what you choose to install.
Does jailbreaking cause instability or battery drain?
Jailbreaking itself generally does not cause stability problems. But you have full access to your jailbroken device, which gives you the power to install software that can cause instability and even battery drain. If you're careful to install well-reviewed, popularly-recommended packages by reputable developers from legitimate repositories, you probably won't run into much trouble with crashes or increased battery usage. You do have to be willing to do a bit of research and troubleshooting though, since you're taking control and responsibility for your device and can install things that cause issues.
Does jailbreaking make my device less secure?
Having a jailbroken device is similar to having administrator power on your desktop computer: you have full freedom to install bad stuff on your computer, but you already know to stay away from installing dubious browser toolbars and sketchy email attachments - instead, you choose to install legitimate software from reputable developers. Use the same reasonable caution when installing software on your jailbroken device. It's generally a good idea to stick to installing software from the default repositories in Cydia (BigBoss, Cydia/Telesphoreo, Dev Team, ModMyi, and ZodTTD/MacCiti), only adding additional repositories if you really trust those additional repositories. If you're interested in technical details, check out this conversation with saurik.
Note: This article is not composed by me, it's from JailbreakQA