An H-1B visa is a nonimmigrant visa used by American companies who wish to employ foreign workers in occupations that require college degrees or their equivalency. Most occupations for which foreign employees use the visa are in medicine, chemistry, engineering, biotechnology and the social sciences. Employees who are granted H-1B visas may only work for the sponsoring American company.
J visa holders subject to the two-year rule are not permitted to remain in the United States and apply for an adjustment/change of status to a prohibited nonimmigrant status (for example, from a J visa to an H visa) or to apply for legal permanent resident status (Green Card) without first returning home for two years or obtaining an approved waiver. Whether you are subject to the two-year rule is determined by a number of factors, including your source of funding and your country's "Skills List." It is not determined by the amount of time you spend in the United States.
A visa is a stamp, sticker, or electronic record sitting inside your passport book that verifies that you’re allowed to stay in a specific country for a certain amount of time. They specify the length of your stay, what territories you may visit, your scheduled date of entry, how many times you may enter the country, and whether or not you’re allowed to study or work during your trip. Not all countries and territories require visas, but it’s best to stay up-to-date on regulations and requirements by doing your research and working with a travel agent. Immigration officials can revoke your visa at any time, and it’s important to remember that they never truly guarantee entry, especially in countries where visas are separate from formal entry permission. An official will likely review your circumstances once you arrive to determine whether or not you may enter.