As implied by its name, copolymer typically uses two materials to improve the drawbacks of monofilament lines, providing less stretch and almost no memory (which lets the line hang straight). Fluorocarbon is a step up, taking a traditional copolymer line and then coating it with fluorocarbon, which makes the line easier to cast, stronger than copolymer, and makes the line practically invisible underwater, though you do sacrifice some sensitivity and abrasion-resistance.