Twins occur when more than one egg is fertilized or a single egg is fertilized multiple times, leading to multiple fetuses. Conjoined twins are identical twins who are joined in some part of the body due to incomplete separation after fertilization. They share the same zygote and sometimes internal organs. Conjoined twins are very rare, occurring in about 1 in 100,000 births. They often do not survive due to the complexity of sharing organs and bodies.