If you transfer title to your house or other assets to a revocable living trust, you can always change your mind and take them back. With an irrevocable trust, you're stuck: It's difficult to regain ownership of anything you gave to the trust or change the original terms. Despite the drawbacks, irrevocable trusts do offer some estate-planning advantages.
Legally, whatever assets you put into a revocable trust are still yours, and you can even make yourself the trustee to manage them. If you put assets into an irrevocable trust, they belong to the trust, not to you, and you must appoint an independent trustee. This may be advantageous if your estate is big enough to face estate taxes -- $5.12 million is the trigger value, as of 2012. As the assets belong to an irrevocable trust, they're not part of the estate and won't be taxed.
One reason to set up a trust is to pass assets to your heirs without going through probate. This works just as effectively with an irrevocable trust as with a revocable one. When you die, the trustee gives the beneficiaries the contents of the trust based on the terms you originally wrote. Another advantage both trust types share is that they're private. A will becomes part of the probate court record, available to anyone, but the terms of the irrevocable trust are known only to you, your trustee and your heirs.
An AB trust is a variant irrevocable trust you and your spouse can use to gain extra estate tax advantages. You and your spouse set up the trust together, and when one of you dies, it splits. Part of the assets go into an irrevocable trust, which is safe from estate tax. The rest goes into a revocable trust controlled by the surviving spouse. Spousal inheritance doesn't trigger estate tax, so if structured properly, an AB trust legally avoids estate tax.
Changes: The Drawback
The drawback to an irrevocable trust is that it's so hard to change. In California, for example, you can petition the probate court to change the terms of your trust, but only if a majority of beneficiaries support the change. The court may agree, provided you can show that the change doesn't hurt the goals of the trust, or that the trust assets have lost so much value it costs more to keep the trust going than it's worth.