[rosenstar]

What exactly does "unlock" mean? Does this term refer strictly to the ability to change the cellular service provider, or does it refer to placing cydia on the phone and opening the phone's file directory? Or is it something else?

Reading this article on 'unlocking' the 4s:
Unlock iPhone 4S Ultrasn0w is Very Imminent - Jailbreak 5.1 / 5.0.1 / 5 / 4.3.5 / 4.3.2 / 4.3.3 / 4.3.4 Untethered - iPad 2 Jailbreak

I have no interest in changing my cellular provider, but I do (obviously) want to jailbreak my device and have access to cydia.

Thanks!

[StaticFX]

Yes. Unlock allows u to use a different carrier.

[Massie]

And for a software unlock like Ultrasn0w, you still have to jailbreak first. We have no definite dates for the release of that either.

(So basically: jailbreaking lets you load apps from outside Apple's closed garden. One of those—Ultrasn0w—is used for unlocking so a phone can be used on another carrier.)

[korp#IM]

Unlock is not just limited to iPhone/iPad either. It can be used to describe any GSM capable device not locked to only work on a specific carrier.