A standard CRB check looks for convictions for offences which may affect your suitability for working with children - sex offences, murder, kidnapping etc.

For people who may have close contact, potentially one-on-one without supervision, with children, an enhanced CRB is required. I believe this includes all criminal matters (spent convictions are listed as such, not ignored) and any charges of which you were found not guilty - but not an arrest record where the CPS didn't prosecute (which was the Soham problem - this is the area which may change soon).