Non-Indian & Non-Bhutanese Citizens
If you are a citizen or national of a Visa Waiver Program(VWP) country holding a passport that meets current requirements, you may not need a U.S. visa for travel of 90 days or less. Current information on VWP participants, guidelines and allowable types of travel is available here. If you do not meet these requirements, you will need to apply for a visa.
If your country participates in the VWP, but you do not have a passport that meets current requirements, or your travel plans are to stay for longer than 90 days, you will require a visa regardless of the purpose of your travel.
If you are from a VWP country and are in India or Bhutan with an urgent need to travel to the U.S., but do not have a passport that meets current requirements, email the Embassy in New Delhi at email@example.com or email your nearest U.S. Consulate General. Please state you are a "third country national" in your email.
Canadians and Other North American Travelers
If you are a Canadian citizen, then in most situations you do not require a visa to enter the U.S. You are required to present a valid passport or other accepted substitute that establishes your identity and citizenship when you travel by air to the U.S. Current rules are available here.