Over cards?

I was reading 'Pocket Pairs and Over Cards By Eric Sprague', but am not sure what is meant by Over card/non over card. Can someone explain.

Look at the flop.

I have AK

Flop is T 3 J

I have over cards (cards that are over those on the flop)

I have QQ with the same flop that is an overpair.

I have AJ with the same flop I have TPTK (top pair, top kicker)

Can someone further expand on this?

Does the term "over card" only apply to what's in your hand. Or can it apply to what's on the board?

Let's say I have jj

the board is 9 k a

are the ka over cards to me? or do I have over cards to the 9? or both?

thanks

I'd say you have an underpair because the majority of the board is over you. But technically you do have an overpair to the 9.

I say underpair, but I could very well be wrong. :D