When you Crit with an attack or spell, you do 50% extra damage to the target. You start out with just a few % Crit Chance depending on your class.

Crit Chance comes in two flavours - Physical and Magical. Physical gives your Physical attacks additional Crit Chance and Magical gives your Magic attacks and spells additional Crit Chance.

If you have 10% Crit Chance, it means there is a 10% chance that your attacks against another target of the SAME level will be a Crit. If the target is a higher level, the chance is reduced, the bigger the level difference, the less chance you will have to Crit.

Protection is the counter to Crit and you also start out with a few % Protection depending on your class. Protection also comes in two flavours, Physical and Magical.

If you have 10% Physical Protection, it does NOT mean you have a 10% chance to avoid getting a Crit, it means your 10% Physical Protection will cancel out the attacker's 10% Physical Crit.

If the attacker has 50% Physical Crit, you would need at least 50% Physical Protection to avoid being Crit by it's attacks.

Some player and pet skills are a little more complicated because they sometimes have an additional built in chance to Crit, so while the player might have 50% Physical Crit, the skill itself might have an additional 30% Crit meaning you would actually need 80% Protection to guarantee it not critting you.