What is Crit? What is CDMG? What's the difference between them?

My Sibyl "always crits". What happens when a party member "crits"?

It says she "always crits". Is there a percentage of time that other party members crit?

If my Sibyl "always crits", I assume that a) she does not need CRIT gems, and b) she should have as much CDMG as possible?

Can someone please explain CRIT to me (as it works in this game) like I'm 5 years old?