If you want to visit the United States for more than 90 days, you must apply for a visa, either a nonimmigrant visa for temporary stay, or an immigrant visa for permanent residence. On the other side, if you come from one of the qualified visa exempt countries under the Visa Waiver Program (VWP), you can visit the United States for a maximum stay of 90 days, without a visa and through the Electronic System for Travel Authorization (ESTA) prior to initiating travel. It is important to know that you cannot work while in the U.S. on a Visa Waiver, because that would be a ground for deportation proceedings initiated against you.
The type of visa required under the U.S. immigration law will depend on the reason of your travel, the period of time and other factors. You can apply for a nonimmigrant visitor visa if you want to enter the United States temporarily for business immigration matters (B-1 visa) or tourism (B-2 visa) more than 90 days.
Nonetheless if you qualify neither for the VWP nor under a B visa, you will need a nonimmigrant visa. Nonimmigrant visas are for workers, business people, or students, who want to taccomplish concrete purposes in the United States for a specific period of time. Petitioners have to prove they have strong ties to their country and intend to depart the United States after their stay.
Immigrant visas are for people who want to live permanently in the United States and become lawful permanent residents, whether or not they plan to seek employment there. Petitioners must first obtain an immigrant visa and must be sponsored by a U.S. employer, citizen relative, or lawful permanent resident. The sponsor has to file a petition with the U.S. Citizenship and Immigration Services (USCIS), before applying for an immigrant visa. Moreover, you can become a lawful permanent resident and permanently living and working in the United States, if you have a relative citizen of the United States or a lawful permanent resident (Family based Immigrant visas). Your relative will need to sponsor you and prove he/she has enough assets to support you.
If you try to enter the United States without the appropriate visa, you may be denied your admission by the Department of Homeland Security (DHS), to the port of entry (airport, seaport or land borders). The Customs and Border Protection (CBP) officer will determine whether you can be admitted to the United States or not.
The State Department establishes the steps and fees for each visa application. You can apply for a visa at a U.S. Embassy or Consulate abroad. You will need to demonstrate that you meet all the requirements to receive the category of visa for which you are applying. The visa points out that a U.S. consular officer has decided that you are eligible to enter to the United States for a specific purpose.