does purchasing an unlocked iPad void warranty?

I"m a little unclear as to all of the conditions surrounding an "unlocked Ipad"

Unlocked is referring to the status of the cellular phone carrier. SO I get to chose who I might want to go with as a cellular provider for web access beyond the wifi component. My thinking is that if I purcahse an Ipad with the cellular function and it is unlocked, I have better options for if and when I choose to use a cellular provider. I read some information that the Unlocked status creates a warranty issue. What does not make sense about that is how are they being sold why is Apple selling new products with no warranty? It seems that the unlocked status is a simple software option issue.

I welcome any and all info on this, as I attempt to educate myself .

Also: what's the differnece between Ipad 4 and Ipad Air, in terms of functional abilities?

Thanks in advance

Nik et Nyt