If you do not have a will, you are considered to be "intestate". If you die intestate, the scheme for distribution set out in certain legislation will come into effect. Specifically, the Devolution of Estates Act governs this issue.
Therefore, if you were to die without a will, or part of your will was invalid, or your will failed to deal with all of your assets, your estate would be divided among the surviving members of your family according to specific proportions based on their degree of relationship to you.
Dying intestate can lead to problems ranging from the taxation of your estate to decisions regarding the guardianship and care of dependant children often leading to the need for direction from our courts which can become unecessarily expensive.