It's basically the same info as on the Green I-94W form .. some personal details, passport details and a declaration that you're not a nazi war criminal etc.
Passport info is compulsory , but address in the US and Flight details are optional (you can log back in to fill them up / change them if you want ... but I never have and I've travelled to the US at least a dozen times on my current ESTA ... you write the address/ flight details on the I-94W form that you hand to the immigration officer anyway)