Personal data is information relating to individuals who can be personally identified from that data (on its own or with other data held). Personal data can be held electronically or physically and includes names, addresses (including email addresses), dates of birth and online identifiers (eg IP addresses).
There is a further 'special category' of 'sensitive personal data' which is awarded greater protection under the law and includes information about racial or ethnic origin, sexual life and physical or mental health or condition.
Criminal offence data (ie personal data relating to criminal convictions and offences or related security measures) is treated separately to personal data and special category special data but is subject to even tighter controls.
For more information, read Data protection.