Your Defense Roll represents how hard it is for both you and your opponent to hit each other. This is calculated differently depending on if you are Dodging or Blocking an attack.
Your Dodge Bonus (DB) is equal to the following:
Proficiency with Dodge + Dexterity Modifier + Armor Modifier + Other Bonuses
Your Block Bonus (BB) is equal to the following:
Proficiency with Block + Dexterity Modifier + Armor Modifier + Other Bonuses.
Armor has a maximum dexterity bonus, you cannot add a modifier that exceeds the limit set by your armor.
Sometimes you are denied your dexterity bonus, in which case you do not add your dexterity to your Defense Roll.
Dodge vs Block
The success of a Defense roll is dependent upon if you are dodging an attack or blocking. If you are dodging a success means you avoid the attack in its entirety. A successful block allows you to apply your armor or shields Damage Resistance Value against the attack. If you are unsuccessful the creature you are fighting has managed to find a weak point in your armor and ignore your Damage Resistance.
For example Seraph is a level 1 Fighter who is wearing Full-Plate. Full plate has an Armor Modifier of 3, and a Damage Reduction value of 7 Seraphs Dexterity modifier is 0, and her proficiency with block is Trained. This leaves her with a +6 modifier to block. (0 from dex, 3 from armor, 2 from proficiency.) She is being attacked by a large spider and needs to roll a 15 or higher to successfully block. She rolls a 11, for a total of 16 meaning her block was successful. The spider still deals damage but it is reduced by 7 because of her full plate. If it reduces it to 0 she does not take any damage and does not take any ill effects from the spiders bite.
Shields provide a shield bonus to Defense rolls when blocking if you use the Raise Shield reaction, if you are using multiple shields, the amount you gain from the second shield is reduced by half.
If you succeed by 10 or more on a defense roll, you manage to perfectly counter an incoming attack and it deals no damage. Referring back to our previous example with with Seraph. If she had instead rolled a total of 25 instead of 17, the spiders attack would be ineffective.
Blocking Area of Effect Abilities
If there is an effect or ability that targets people in a linear fashion, such as an Axe's Sweeping Strike, or the Lightning Bolt spell you may attempt to stop the attack from proceeding past you. For physical attacks such as the Axe, a successful block stops any attacks that would proceed to hit any subsequent allies unless otherwise stated. For area of effect spells that deal damage in a line, or cone you may attempt a block instead of a reflex save. If you are successful the attack does not affect targets behind you, but you my not make a reflex save to reduce the amount of damage taken or to avoid any ill effects. This does not prevent from making any other types of saves associated with the spell or ability.
Saving Throws :
When targeted by some spells and abilities you make a saving throw to throw off the effects. Saving throws are calculated as follows :
Save Proficiency Bonus + ability modifier + miscellaneous modifiers
Saving Throw Types :
The three different kinds of saving throws are Fortitude, Reflex, and Will.
Your bodies ability to throw off ailments such as disease, poison, or the life sapping energy of necromancy spells. You apply your constitution modifier to your Fortitude save.
You speed and ability to remove yourself from dangerous situations. Situations include bracing yourself from an incoming fireball, jumping out of the way of a boulder and staying upright in an earthquake. You apply your dexterity modifier to your Reflex save.
Your mental resistance to spells that threaten to warp your mind. You apply your wisdom modifier to your Will save.
Damage Resistance and Energy Resist
Damage resistance reduces the amount of damage taken from physical attacks, though some types of Damage Resistance are weak to specific attacks generally noted in a creatures or armors stat block of DR/Blunt, or DR/-. Any damage types listed bypass the Damage resistance.
Energy Resist functions in a very similar way except it only protects against any spells or abilities with that energy type in the description. If multiple are listed the damage is assumed to even split between them.