According to the US government website, “Generally, a citizen of a foreign country who wishes to enter the United States must first obtain a visa, either a non-immigrant visa for temporary stay, or an immigrant visa for permanent residence.” If you are moving for a short-term assignment, you will need a temporary worker visa. Long-term or indefinite postings fall within the immigrant category. The US government offers a visa FAQ page which covers the basics about immigrating to the United States.