A record of a person’s criminal convictions and cautions.
There is also record of any arrest, out of court disposal (including youth cautions) and conviction on the police national computer (PNC). This information is only deleted when a person reaches 100 years old.
Usually when people refer to someone having a criminal record it means they have unspent convictions. This is because people who have committed offences are considered rehabilitated after a period of time (see Rehabilitation of Offenders Act). When people apply for most jobs they only have to tell their employer about unspent convictions (and do not have to tell the employer if they have spent convictions).
It can be difficult to understand when criminal convictions and cautions are spent. There is a quick look guide here.