OK the damage is described as the number of dice rolled and what kind of dice(number of sides) . A regular die is six-sided and is described as d6, so 10d6 would be the random roll of ten six-sided dice. the results would vary between 6 and 60... and eight sided die would be d8 and a ten-sided die would be d10, etc.
Priest spells come from your diety and you have no control over what you get or when you get them - but on the other hand - you do not have to buy them or find them and there is no chance of not learning them. Usually you can learn more priest spells than arcane spells - but it depends on your level and your wisdom stat.