The Difference between ESTA and I-94W is mainly their purpose and timing.
ESTA is an electronic travel authorization that must be obtained online before traveling to the United States, while Form I-94W was the paper arrival and departure record previously used by Visa Waiver Program travelers.
Under current U.S. entry procedures, eligible VWP travelers are generally required to have an approved ESTA, and the old paper I-94W card is no longer part of the usual process.
Therefore, if you are eligible for the Visa Waiver Program, ESTA is generally what you need, and preparing a separate I-94W is no longer part of the usual entry requirement.
If you are planning a trip to the United States, please check your passport information and travel purpose, and apply for ESTA before departure.