EdT and EdP refer to the amount of oil in the formula - If I recall correctly, EdTs are 4-8% oil, EdPs are 8-12 (possibly more) % oil. So generally EdPs last longer as they are more concentrated.
1oz., 1.7oz., and 3.4oz are imperial sizings (ounces). 1 oz = 30 mls = ~300 sprays. 1.7oz = 50 mls, 3.4 oz = 100 mls.
The original Black Orchid only comes as an Eau du Parfum (EdP), while Voile de Fleur is a 'lighter' version of the original, and comes as an Eau du Toilette (EdT). They smell different, so dont buy Voil expecting Black Orchid!