Trusts are set of rules related to assets. Trusts that can be changed are called revocable trusts, and trusts that cannot be changed are called irrevocable trusts.
Up until very recently, trusts were necessary to avoid or decrease estate taxes. Trusts help people avoid probate for the assets in the trust. Trusts can also set up rules on how assets are used by heirs. Finally, trusts are used to gradually distribute assets to people who might struggle if given a large sum of money at once or who may have creditors who would try to seize money if it were readily available.