If you die without making a Will, you will have died Intestate.
This means that your Estate (all that you own, such as; property, possessions, cars, cash, works of art, jewellery, savings, investments, insurances and your personal belongings), will be distributed in accordance with the Intestacy Rules as laid down by law.
These rules of intestacy are applied when the deceased person has not left a valid Will. They are designed to provide first for a surviving spouse (or civil partner), the amount allotted depends on whether the deceased had children or not. If the deceased was not survived by spouse or children, the rules then go on to set out the order in which other relatives may inherit the the Estate. If there are no relatives, then the entire Estate will pass to the Crown.
Please see the current Intestacy Rules for more details.