Fraternal twins, also known as dizygotic twins, occur when two separate eggs are fertilized by two different sperm cells during the same menstrual cycle. This results in two genetically unique siblings who share about 50% of their DNA, similar to regular siblings born at different times. Unlike identical twins, monozygotic twins, who come from a single fertilized egg that splits into two, fraternal twins can be of different sexes and may not look alike. Their differences arise from the fact that they develop from separate eggs and sperm, leading to varied physical traits and characteristics.