A:
without going into too much detail:
mono - most stretch, floats, semi transparent
flouro - low stretch, sinks, completely transparent
braid - zero stretch, floats, not transparent at all (usually)
each has their won strengths and weaknesses. Flouro is great for clear water as well as fishing deep beacuse it sinks. Braid is awesome for frogging and flipping/pitching heavy cover because of it's small diameter and zero stretch for great hooksets. Mono is a great all-round line that I always have spooled up on at least one rod because it can be used for almost anything in a pinch.