Usually, foreigners must have a visa in order to enter the U.S. The only exception is the Visa Waiver Program for nationals of certain countries.
Persons who cannot enter the U.S. without a visa due to nationality or purpose or duration of the stay have to apply for a visa.
As a business traveler (B-1 Business Visitor) or tourist (B-2 Tourist Visa), it allows you to stay in the country for up to 180 days at a time or to enter the country multiple times during a year. Usually those visas are valid for ten years, but can also be limited to a shorter period of time. This means that visa holders can, for example, spend 180 days in the U.S. over the course of the next ten years. Please note, however, that, even with this type of visa, entering the country frequently can lead to questions and/or problems at the border. This will especially be the case if the border officials suspect that your center of vital interest is shifting to the U.S. or that you are planning to immigrate into the U.S.