The easiest to get tourist authorization and only citizens of selected countries are able to apply.
ESTA is valid for 2 years during which, you are allowed to enter USA multiple times for maximum of 90 consecutive days. It allows you to travel for leisure or business but not to work or study.
Disadvantage compared to regular tourist visa is that in case you will be denied entering the country, you are not able to dispute this decision. However if you completed the application truthfully, this is happening very rarely.