The ESTA USA system is an electronic system that is used to pre-authorize people who wish to travel to the United States under the Visa Waiver program. The ESTA USA is a confusing system, and a lot of people mistake it for a Visa or assume that if they have an ESTA then that means they are guaranteed to be able to enter the country, this is not the case. The ESTA, essentially, replaces the yellow cards that used to be handed out prior to flights landing with a new, pre-flight check process.
The purpose of the ESTA is to do some preliminary checks that will speed up the process of passing through Customs and Immigration at the border. You must have an ESTA before you attempt to travel to the United States, but all the authorization does is confirm that you are eligible to travel. If you plan on staying a long time, working while you are in the country, or otherwise doing something that will need a Visa, then you will need to apply for a Visa separately.
The good news is that applying for an ESTA is a simple process. You can complete the application form online and in most cases, you will have a response to your application within minutes. In some cases, the checks may take a little longer. This may be the case if you have moved around a lot or if you have a very common name and it takes a while to confirm your identity. Because of the potential for delays, the US government recommends that people apply for an ESTA as soon as they start planning their trip. You can travel multiple times during the validity period of the ESTA, which makes it good value for money and a relatively hassle-free thing to sort out.