Clarification for Eddie & Nailit:
When caution were first introduced, one of their big 'selling points' (in Parliamentary debates) was that they would NOT create a criminal record.
However when the Criminal Records Bureau was set up, cautions became included within their system and thus appeared on CRB checks. So they weren't meant to be part of a criminal record but, de facto, they became so.
As I've written above, the Disclosure & Baring Service (which has replaced the CRB) now filters out cautions from DBS checks after 6 years (except for certain specified offences, such as sexual ones).