In order for a foreign national to be authorized to work in the United States, you must have either a Lawful Permanent Resident Card (Green Card) or an Employment Authorization Document (EAD). Generally, in order to obtain an EAD, you must first obtain a visa that permits work authorization. There are additional classifications, such as Asylum and Temporary Protected Status, that also allow for work authorization. Certain instances where a pending application or special circumstances may allow you to obtain work authorization as well.
There are many different types of work/business visas that allow for temporary employment authorization. Some visas, however, such as the B-1/B-2 Visa, do not permit employment. Some foreign nationals on certain educational visas may also be permitted to work under limited circumstances. There are also various ways to obtain Lawful Permanent Residence.