Saving Throws

I have been playing BG for years and I am embarrassed to admit I still don't understand how saving throws work. I know they are great and they can protect you from nasty magic and the more the better but that's about it.

I would love a simple explanation of how they work. Is there a limit to how low/high they should be? Will great saving throws make me immune to negative status effects?

Basically I'm making a fighter/cleric and I'm trying to decide between dwarf and half-Orc. Are the shorty saves really that big a deal?

Thanks!!