In most pregnancies, a single embryo develops in the uterus, but sometimes there can be twins, which means that two embryos develop together.
Most twins are Fraternal or Dizygotic twins, meaning they originate from two separate eggs that are fertilized individually. While only a minority are: Identical or Monozygotic twins, which means they come from a single zygote that quickly splits into two separate groups of cells. Fraternal twins are from two separate eggs that are fertilized by different sperms, so they are completely separate genetic makeups. They don’t look any more or less alike like regular siblings, although the resemblance can still be very close.

Fraternal twinning happens at the rate of 10/1000 births worldwide, most of the times; fraternal twinning happens when the ovaries release two eggs simultaneously, which is called hyper ovulation, instead of releasing one egg at a time. Research suggests that some mothers of fraternal twins might produce overabundance of a hormone called Follicle stimulating hormone (FSH) which stimulate the growth of ovarian follicles. Mothers of fraternal twins tend to be taller and heavier on average, which shorter and more frequent menstrual cycles all of which are characteristics of having high follicle stimulating hormone. Because follicle stimulating hormone gradually rise with age, women become increasingly likely to give birth to fraternal twins once they are 35 years or older, and this explains why mothers who have given birth to fraternal twins once are more likely to do it again.
The likelihood of having fraternal twins from hyper ovulation is thought to be genetically linked, while no specific gene has been identified yet.

Identical twins are even less common than fraternal twins, happening at a rate of 4 per 1,000 births worldwide.
Identical twins come from a single zygote splitting to form two separate embryos with identical genetic material. This split can happen at any time between the first 13 days of development, and how & when the division happens affects how the identical twins share space and resources in the uterus. Because identical twins have identical DNA, they share a lot of physical traits that have a strong genetic basis, like sex, hair and eye colour, blood type, and other physical features. In fact, subtle differences between identical twin babies often show how the environment, even the environment of the uterus, can also affect development.