It is the process of removing some of the artificial limitations placed by Apple on their devices. Mainly this enables users to download additional applications not approved by Apple, themes etc., that are not available via iTunes. Typically a jail broken device can still use iTunes. It sounds so much better than Rooting an Android (official terminology). If you want to know more read the Wikipedia article.
I am not aware of its impact on carrier unlocking and given the iTWire article on this I will not comment further.
I don’t purport to be an expert here so this article is admittedly a little light on detail.
I know that evad3rs developed it and that it is available from evasiOn . It runs on Windows, Mac and Linux. It is compatible with iPhone 4 and 5 (not 2 and 3), iPad (not first generation) and iPad Mini but not iPod Touch (first to third generation) and Apple TV (third generation).
I know that the QA segment is showing several issues – things that may have gone wrong so back up any device that you jailbreak.
And I know I won’t win any friends at Apple for reporting this.