Irrevocable trusts are exactly as the name implies, they (generally) cannot be terminated after they are finalized. This is different than a Revocable trust which you can terminate any time you wish and only become irrevocable after the death of the grantor (the trust maker).
There are several types of irrevocable trusts, and they serve different purposes. Two of the most common reasons to create an irrevocable trust are (1) to reduce taxes; and (2) to protect your property. There are various types of trusts with different names that won't mean much to most people, but the essential take-away is that there are trusts that can be created to reduce taxes, to protect assets, or to care for people that are unable to manage their own money.
Some examples of various irrevocable trusts are:
A Bypass trust is used by spouses to reduce estate taxes when the second spouse dies.
There are various types of Charitable Trusts (Remainder/Lead/Income).
A Generation-Skipping Trust are designed to reduce estate taxes for wealthy families.
A Life Insurance Trust reduces estate taxes by removing the proceeds of life insurance from a taxable estate.
A QTIP trust is used to postpone taxes until the second spouse dies.
A QDOT trust is like a QTIP trust, except it is used when one spouse is not a United States Citizen.