Need help understanding "credit"

Hi,

I'm from Argentina and I'm trying to figure out the cost of distance
learning...

All the Universities I've seen so far give you the cost by credit, but I
don't know what that means.

Furthermore, one of the Standford distance-learning programs said about
$1100 each "unit", and that a degree is 45 credits. Is unit=credit?

If a credit is one "course" (in Argentina we call a yearly course: Materia),
and it costs, say, $500.... how does that correlate to the Monthly fee I
need to pay?

TIA

Fernando Cacciola