The H1B is a work visa for non US citizens who want to work in the United States. Although there is no such thing as an H1B Green Card, the H1B is often used as the first permit non citizens use in order to come into the United States since it is quicker to get an H1B visa than a green card.
As an individual, it is not possible to apply for an H1B visa. Only an employer can apply for an H1B visa for their employee. Once an employer has found an employee and has decided on a start date for the job, they can begin applying for the H1B visa as early as six months before the desired start date.
In general, H1B visa jobs are considered to be specialty jobs. In other words, H1B visa jobs are jobs that require a high degree of specialized knowledge of one kind or another. In most cases, the equivalent of a job-relevant 4-year US Bachelor's degree is required, or a post-graduate degree of some kind. Certain positions, such as a lawyer or doctor must also be licensed for their practice in the state in which they intend to practice, by passing the bar exam or other such examination for other positions and vocations.
In order to obtain an H1B visa, you must first have secured one of the many H1B visa jobs. After getting the job, you will need to have your employer apply for and complete the H1B visa form. As previously stated, individuals cannot apply for the visa; only employers may apply for the visa. One advantage to having an H1B visa is that once you have received the special work visa, you will be eligible (and highly likely to obtain) a green card for more permanent work in the United States.
There are also many other kinds of visas for travelers coming to the United States. Student visas, visitor visas and immigrant visas are all legitimate ways to come and spend time in the United States. Work visas are a great way to come and experience life in the United States, and finding a company to sponsor you for an H1B visa is a great opportunity to gain entrance into the US much faster than you might with a Green Card or other kind of work visa.