An irrevocable trust is one that can't be taken back or "revoked" after it's established.
Such a trust can be set up to operate in a number of different ways, depending on the trustmaker's goals and the trust beneficiary's needs.
It can provide a steady stream of income to a beneficiary, which is sometimes called a unitrust. Or proceeds can be distributed as a percentage of the trust, which can rise or fall with the trust's investment performance. Or the trust can be set up to allow the trustee to use discretion to distribute proceeds for whatever a beneficiary needs.