There is are a couple of different damage types and each has different effects and mechanics. Having knowledge of these damage types will allow to make more informed decisions when squad building.
When two units match their attacks at the same time, they will spark. Spark does more damage in that matched hit. You can boost this damage by using Spark Damage Up, Spark Vulnerability, Spark Critical buffs.
When 2 Omni units of the same element spark, they do 1 Cross Spark, this activates a passive for units of the same element, called Resonance. Cross Spark is still spark damage.
When you unit does a critical hit (it's by chance) you do additional damage. You can raise this damage by Critical Damage up buffs and/or Critical Hit Rate up to get more chances at doing critical damage
Elemental Weakness Damage
This is extra damage you do to a weaker element and you can amplify it further via EWD damage up buffs. There are many variants like Fire Elemental damage up to being general just by being called Elemental Weakness damage up