The bombard value is measured against the defense value to determine if borbardment is successful, in which case a hit point is lost. I *think* this works exactly like regular 1 on 1 combat, i.e. taking into account the defenders terrain etc.
Bombardment units have a second value called the rate of fire, which is usually 1. Artillery has 2, though, meaning they get two chances of hitting and often enough make the defender lose 2 hitpoints with one attack. Bombers and Radar Artillery even have a rate of fire of 3, iirc.
I don´t think it´s of any importance in what kind of terrain the Bombard unit is placed. As said, I *think* it´s handled quite similar to normal combat, where this is not an issue, too.