Anything that identifies a living person, directly or in combination with other information you hold.
Personal data is any information relating to an identified or identifiable living individual. A name and an email address obviously qualify. So do a national identity number, a photograph, a vehicle registration, an IP address, a CCTV image, and a customer reference that can be tied back to a person using other information you hold.
The Act also recognises special category data, which needs stronger protection and usually explicit consent or a specific legal basis. That covers racial or ethnic origin, political opinions, religious beliefs, trade union membership, physical or mental health, sexual life, and genetic or biometric data.
A useful test: if you could use the information, alone or with something else in your systems, to work out which person it refers to, treat it as personal data.