Every person entering the United States from a previously visa exempt country is required to have a ESTA US visa in order to gain entry into the country. Currently the visa is only applicable to travelers arriving by sea or air and does not apply to landed borders. If you have any other type of visa such as a work or student visa, you are not required to apply for an ESTA visa.
It is very important to understand that while entry into the U.S. will not be granted without this visa, neither does the visa guarantee entry. ESTA basically acts as a screening tool that is used to determine a traveler’s eligibility to enter the country. It is still up to the customs officer at the border of entry to decide whether a visitor meets with the necessary criteria for entry into the country and even if you have an ESTA visa, you may still be denied entry into the country for reasons such as a criminal history or not having the available funds to stay the expected time in the country without working.
The ESTA visa allows for a maximum period of up to 90 days. However, you will need the visa whether you are planning to spend one night in the States or the maximum. The visa is valid for a two year period and allows for multiple entries into the country. This means that you can return to the United States repeatedly within a 2 year period without having to reapply.
It is however important to note that you will not be able to simply cross a border and immediately return to the country in order to comply with the multiple entry rules. The general rule is that you will need to stay out of the U.S. for an equal period as you were in the country. For example, if you stayed for 30 days, you will have to remain out of the country for more than 30 days before you can return.