Assuming that the producer is honest, "organic" indicates that the sources from which the essential oils were extracted were organically grown, i.e. no pesticides, synthetic fertilisers, etc. that are used in conventional agricultural methods.
I really don't know what sort of impact organic sourcing would have on an essential oil, but the absence of certain chemical compounds (from fertilisers or what-have-you) may have an at least subtle effect on the actual scent. The brand may also be less consistent, since the growing conditions would be a bit more at the mercy of natural influences. I believe the difference would be quite small, however (it IS, however, nice to know that you aren't contributing to dumping more crap into the environment, which may eventually find it's way into your drinking water).
100% essential oils should indicate that the essential oil is not diluted in a carrier oil (e.g. almond, jojoba) or other solvent (e.g. alcohol); it COULD, however, be combined with some other, cheaper essential oil, so unless the label state that something is 100% essential oil of [whatever], there is the possibility that if "whatever" is expensive (e.g. lemon verbena), it may not be pure "whatever"