In 2004, a grand total of 946,142 people gained entry to the United States of America as legal immigrants. Each one of them applied for and received a visa of some sort. Since the USA is such a popular destination, there are a variety of visa types that you can apply for which will allow you to make an extended stay in the United States. This is a quick guide to the various different types and what makes them different.
One Simple Rule
If your situation requires a visa you must apply for it from your home country. You will not be able to change your status1 or apply for a visa from within the United States. If you have entered the country under the Visa Waiver Program (see below) you will have to leave the US and apply for a visa from your home country. The application process can be lengthy, so be certain you have left yourself plenty of time before you intend to travel, and will probably require a visit to your local American Consulate. A general guide to application and processing times can be found on the USCIS2 website.
That all depends on what you want to be in the USA for. For example, if you're a citizen of one of the participant nations in the Visa Waiver Program and you're only coming over on holiday, you need not apply for a visa at all. If you want to spend more than a few weeks in the States or if you want to work while you're there it's a rather different story.
If the company you work for has decided it wants you in the USA for any appreciable amount of time, it will arrange this for you. You'll be entering the USA under a work visa; the exact type depends on your situation and qualifications. These are also classed as non-immigrant visas, which means that although you can renew or extend most of them to allow yourself more time in the US they do not allow you to apply for a Green Card3 or apply for citizenship4.
L-1 - Your company is transferring you from one of their offices outside the US to one inside. Typically, multinational corporations will indulge in this; all you need to do to qualify is have a year's continuous employment at the same company and be either a company executive/upper level manager (they get the L1-A) or have specialised knowledge (the L1-B). The initial stay is 7 years (L1-A) or 5 years (L1-B) and is renewable in 3 year slots.
H1-B - A US company wishes to hire you and have you work in the USA. Your company will need to demonstrate a couple of things. First, you need a specialised occupation. Next, you either need a degree or equivalent years of experience in that occupation. The formula they use is three years' real world experience is equal to one year of college study5. Additionally, your prospective employer will have to show that your wages will be equal to or higher than those of a US Citizen doing the same job, that your benefits will be the same and that they have tried and failed to find a US Citizen to do the job first. They're also not allowed to employ you if they'd be doing so to work around a strike or lockout.
You get to stay for six years maximum, but the visa can be renewed. Employers do so about six months in advance. Annually, there are only 65,000 of these visas made available and they go pretty quickly. If you arrive in the USA with your spouse and kids, be aware: this visa does not allow them to work.
H1-B1 - If you're living in Chile or Singapore, this is the 'New Hire' visa you'd come in on. This one lasts three years but can be renewed indefinitely.
E-3 - For degree-qualified Australians, there is no limit to the number of E-3 visas the US will issue. Your spouse gets to work and you can renew the visa as often as you need to. This is just another reason they call Australia the 'Lucky Country'.
F-1 - You're a student with a place at a US institute of learning. Your visa lasts the length of your stay plus an optional 12 months at the end for 'operational or practical training'; in other words, to put your education into practice. This visa allows you to work; part-time during the academic year, full-time during vacations.
J-1 - You're a trainee, and you're being paid to train, or you're a researcher6. On the expiration of this visa you need to be in your home country for two years before re-entering the USA.
This range of visa is designed to allow foreign nationals to enter the USA following, or leading up to, a marriage to a US citizen.
K-1 - The Fiancé(e) Visa - you are planning to marry a US Citizen on US soil.
K-2 - You are the unmarried child of a K-1 Applicant and are under 21.
K-3 - You have already married a US Citizen and are living abroad or apart from them waiting for an adjustment of status.
K-4 - Your parent is a K-3 Applicant.
The K series of visas were created under the LIFE act, which broadly speaking tries to prevent families being separated while things like adjustments of status are made. As is the case with other visas, you have to apply for them outside the US and they allow you to stay in the country while you file the paperwork that will let you become a Resident Alien, the first step in becoming a US Citizen.
O-1 - For people of extraordinary ability. To meet this criteria, you need to be at the very, very top of your field8. Its purpose is primarily to allow genius of one sort or another easy access into the United States.
P - Internationally recognised athletes, entertainers or entertainment groups use this visa to enter the USA to compete, tour, gig or otherwise perform9.