The main difference between a revocable and irrevocable trust is when the person who funds the trust gives up control of the assets. With a revocable trust, you maintain control of your assets until you pass away, at which time the trust becomes irrevocable.
With an irrevocable trust, you give up control of of your assets as soon as you put them in the trust.
Generally, you will use a revocable trust to more efficiently distribute assets. While an irrevocable trust is often used for asset protection or tax planning.
If you are interested in more information on trusts, check out my article on trust basics here.