Revocable or Irrevocable Trust? What is the difference?

Here in beautiful California, we do not use the Irrevocable Trust as much as in the lesser 49 states, but it has uses, especially as we get older. But everyone with any property should have a Revocable, or Living, Trust.
A Living Trust is an estate planning tool for regular people. It lets you skip over the onerous Probate process with the courts. Probate can take only a year if you are lucky and will probably cost about $1,000 in fees in you do it yourself. If you hire someone to do it, the state regulated fee starts at 4% of the gross estate (not net of mortgages or other debts). It is a nice step to skip by using a Trust to put your named Trustee in charge instead of the judge.
The Living Trust also lets you haunt your heirs from beyond the grave by making rules for how the money is used. With a Will, the kids get their inheritance at 18. With a Trust you can direct the spending (like, college only and not a sports car) and give them the money when they are older (or even keep it in Trust their whole life if that is what they need).
A Living Trust is also used to plan to keep Medi-Cal from taking back the money they paid on your behalf after you die. Medi-Cal will pay your nursing home if you cannot afford it (maybe), but once you are both deceased (you and your spouse), Medi-Cal wants the money back from your estate rather than letting you leave it to your children.
A Living Trust is like a cardboard box. We put things in, and we take things out all we like while we are alive. You pay the taxes the same way you always did, you preserve the stepped-up basis advantage so the heirs pay less taxes and you will hardly know it is there.

An Irrevocable Trust is more like a pirate treasure chest. You put all your extra treasure in there, lock it up, AND THEN GIVE SOMEONE ELSE THE KEY! This naturally is uncomfortable no matter how much you trust the Trustee you put in charge. So why do it?

The Irrevocable Trust is used most often to protect the assets from law suits or from the government. If you are in a profession that gets sued a lot (like a brain surgeon or something) or just a person who gets sued a lot (like a really, really bad driver), then you might lock away those assets out of your control so that no one can get to them.
What regular people use them for is Long Term Health Care planning. About one-third of us will end up in a nursing home, which costs about $10,000 per month. Medicare will NOT pay for it beyond the first couple months, and usually not even that. You are expected to use your own assets until you are broke enough to qualify for Medi-Cal. You qualify (generally speaking) when you are down to just your retirement plans, home and personal items and a small amount of money (depending on whether you are married or single).
If you have too many assets to qualify (like a second house, cash, stocks and bonds outside of your IRA/401k) then you can put those assets into an Irrevocable Trust (our treasure chest) and once those assets are in the Trust for 30 months they will become invisible to Medi-Cal (as if we buried the treasure chest on an island). You could always give away those assets, subject to the same 30 month exclusion rule, but that often has bad tax consequences for the recipient because they get your tax basis along with the gift; if instead they inherit then the heir gets a stepped up basis which usually means much lower taxes when they eventually sell the asset.
Once you have put those assets in the Irrevocable Trust you no longer have control over them. The control you are limited to is naming the Trustee (so you can change out a bad one), and what happens to the assets after your untimely passing. The Trustee of course knows that the Trust is for your benefit and can, at his discretion, give you money from the Trust. It must be at his discretion - if you have the right to demand money then you have control and can demand money to pay for your nursing home care, thus disqualifying you for Medi-Cal.
The use of the Irrevocable Trust is part of a plan to keep your money in the family instead of giving it to the government who will probably just blow it on something stupid.
Please note that this all applies in California only. Away from here the rules get far more restrictive.