I really need to know. Please tell me.

1. How does it work? What is UserLand?
2. Can it be done to all Apple devices(any)?

Unfortunately those devices are the same ones that Apple can prevent you from downgrading unless you’ve got a backup of your personalized SHSH blobs. Unless you’ve backed up your SHSH blobs for vulnerable firmware versions, you’ll lose the ability to use the current Spirit jailbreak if you accidentally upgrade.

3. What are SHSH blobs?
4. Will it change if I leave my iPhone 3GS aside for a long time?
5. Can it void my warranty?

6. Can I fix my iPhone if the operation goes wrong?
7. I heard iTunes 9.1.1 blocks the Spirit Jailbreak. If I connect my iPhone to the new iTunes will I never be able to use Spirit Jailbreak?

http://spiritjb.com/