HowTo: Jailbreak & Unlock

Since there is a big jailbreak community but nevertheless many people still have many questions, I thought a big HowTo about Jailbreaking and Unlocking would be nice: 

Infos
Which iDevice do I have? 
Can I jailbreak/unlock my iDevice? 
Jailbreak-News

Downloads 
Firmwares

HowTos
Jailbreak on MacOS / on Windows
DFU/Recovery-Mode 
Unlock on 4.2.1

Any other information needed can be found via Google :D

Have Fun!