A trust is a legal arrangement that provides for the ownership, management, and distribution of property. Think of a trust as a box into which someone places property. The person placing the property into the trust is known as the grantor of the trust. The person that oversees the property in the box is the trustee. Finally, the person who receives the benefit of the property held in the box is known as the beneficiary.
An irrevocable trust is simply a trust with terms and provisions that cannot be changed by the grantor. This is distinguished from a revocable trust, which is commonly used in estate planning and allows the grantor to change the terms of the trust and/or take the property back at any time.