IWD2 uses 3E saving throws, which are categorized as follows:

Generally, most classes have one or two good saving throw categories and one bad saving throw categories. A few classes, like the monk, have good saving throws across the board.

All bonuses to saving throws stack.

When a character attempts to roll a saving throw, a random number from 1-20 is generated. The character's saving throw bonus, and modifiers, are applied to that roll. If their sum is equal to or greater than the DC (difficulty check) of the attack, they successfully save. While some saving throw DCs are arbitrarily derived (a dragon's breath, for instance), most spells are derived using the following formula:

Spell Level + Caster's Ability Bonus + 10 = Spell DC.

E.g., Marfain Vaer casts a fireball. She is a sorcerer with a 16 Charisma. Since fireball is a 3rd level spell, and her ability score modifier is +3, the DC of her spell is 16 (3 + 3 + 10 = 16).

In combat the target must roll d20, then add his or her saving throw + ability score modifier. E.g.: Marfain Vaer casts Tasha's Hideous Laughter at Aganti Devasquel. The dark elf sorceress is casting a 2nd level spell and she has a 20 Charisma. Ergo, the Will DC to resist her spell is 10 + 2 + 5 = 17. Aganti the halfling rogue is 5th level and has a measly +1 Will save. However, he has a +1 to all saving throws due to his race. Unfortunately, he has a 9 Wisdom, so he has -1 on top of that. +1, +1, -1 = +1. Aganti must roll a 16 or better on his d20 roll to match (and thereby overcome) the spell.

Saving Throws increas as PC's level up, but not at a 1:1 ratio. Generally, each character class has one or two good saving throws and one or two "bad" saving throws. For example, rogue characters have good Reflex saving throws, but poor Fortitude and Will saving throws. At 5th level, a rogue has +4 to his or her Reflex saves, +1 to his or her Fortitude and Will saves. Fighters also have crappy Will saves, which is why in IWD2, fighters with low Wisdom scores can get killed very quickly in battles with clerics and wizards using Hold Person or Mental Domination

Characters can get pretty high ability score bonuses through the use of additional ability score points and spell enhancements. If Marfain Vaer were 8th level, she could have two additional points in Charisma, raising it to 22. If she cast a spell like Eagle's Splendor (which is not in IWD2 -- sorry), she could raise it to 26 or 27. That's a +8. So, yes, saving throw DCs can get pretty high.