A revocable trust is a trust whereby provisions can be altered or canceled dependent on the grantor. During the life of the trust, income earned is distributed to the grantor, and only after death does property transfer to the beneficiaries.
An irrevocable trust can't be modified or terminated without the beneficiary's permission. The grantor, having transferred assets into the trust, effectively removes all of his rights of ownership to the assets and the trust.
The simplest difference between the two is that assets remain in the grantor's estate in a revocable trust but move out of the estate in an irrevocable trust.