CRB stands for Criminal Records Bureau. CRB is the term which has come to refer to a check made by the Home Office/Police on the suitability of a person to be able to work with children and vulnerable people. This can be on an employed basis or even a volunteer basis.
There are three levels of Disclosure: Basic, Standard and Enhanced. Depending on the type/level of check requested, it may contain details of spent and unspent convictions, reprimands, warnings and where appropriate, local police records. If a Disclosure contains no record of criminal convictions, it is generally considered to be a testament to good character and conduct.
Once a CRB check is completed, an Official Disclosure Certificate (also referred to as a disclosure document) is issued by the CRB showing the results of the check.
Most organisations where a CRB is required will ask you to undergo a CRB check even if you have had a recent CRB check. This is not always the case but is considered good safeguarding practice.