Dying without a will - known as dying "intestate" - means you have no say over who receives your assets, and can leave your heirs and the court system the complex and costly job of wrangling over who should get what.
Your assets go into what's called "probate" - an expensive and drawn out legal process which determines who inherits your estate, and can take anywhere from a few months to a few years, depending on how complicated the estate is.
So-called intestacy laws vary considerably from state to state. In general, though, if you die and leave a spouse and kids, your assets will be split between your surviving mate and children. If you're single with no children, then the state is likely to decide who among your blood relatives will inherit your estate.