When a person dies, their assets are referred to as their "estate." In order for ownership of those assets to pass properly, an Executor or Trustee must go through the steps of gathering the assets and distributing them as provided in the decedent's will or trust. This is referred to as Estate and Trust administration.
If a deceased person had no will, they are considered "Intestate." If they had a will, they have a "Testate" estate. In either case, if they have assets in excess of a threshold specified by Illinois law, a probate proceeding is likely to be required.
Often a deceased person used a trust to plan for their estate. If the decedent's assets were transferred to their trust prior to their death, a probate will not be required. There are still many steps for the successor trustee to take - just no requirement for court supervision.
In either case - probate or trust administration - it is best for the family (or friend) of the decedent to contact an attorney soon after the death. There are many executor/trustee responsibilities that can be missed or forgotten among the other details of arranging the funeral and burial.