All you need to do is go to the help pages on easyJet.com, select the "Our FAQ's" link at the bottom of the page and then input in the search field "What ID do I need?"
You will then receive a reply saying:
Domestic flights:
Passengers are required to provide photographic ID at check-in all flights including domestic services.
International flights:
The documentary requirements for passengers on international flights within the European Union are a valid passport or national identity card supported by any other relevant documentation such as visas where applicable.
For all other international flights out of the Eurpean Union a passport is required.
If you are unsure what constitutes a valid ID we recommend you contact our customer service department on 0871 244 2366 (calls cost 10p per minute; calls from mobiles and other networks may cost more) for further information.