An ESTA approval number gives the applicant approval to travel to the US, however, it does not give the applicant the right to enter the US - this is down to the US border control authorities. Of course, it is rare for a traveller to be stopped at the border, but it can happen. An ESTA does NOT give the applicant the right to reside or work in the US, and it is only valid for a maximum of 2 years from date of issue (or until expiry date of passport, whichever is sooner).
Citizens of the following countries only are eligle to apply for ESTA under the Visa Waiver Program:
Republic of Malta