identical twins is formed with 1 egg and 1 sperm, then split into 2.can be 1 or 2 placenta, or 1 or 2 sacs.
fraternal twins is formed when 2 eggs r released, formed with 2 sperms, is as gd as 2 pregnancy at one go. thus they can be a boy and a girl or even identical but they may not look as similar as identical twins.
thats my understanding....