For Canadians and Bermudians, the entry period is normally for a maximum of six months.
For travellers under the Visa Waiver Program, the entry period is strictly limited to 90 days: Andorra, Austria, Australia, Belgium, Brunei, Chile, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Japan, South Korea, Latvia, Liechtenstein, Lithuania, Luxembourg, Malta, Monaco, the Netherlands, New Zealand, Norway, Portugal, San Marino, Singapore, Slovakia, Slovenia, Spain, Sweden, Switzerland, Taiwan and the United Kingdom.
Obtaining a visa
For the rest of the world, or for those who don't fit the profile of a Visa Waiver Program entry the visa application fee is a non-refundable $160 for visas that are not issued on the basis of a petition and $190 for those that are. To obtain a visa, face-to-face interviews at the nearest US embassy or consulate are required for nearly all nationalities. Since waits for interview slots and visa processing can add up to several months, you must start researching how to obtain a visa well in advance of your planned departure date.
Common US Visa/Residence Statuses
B1: Business visitor
B2: Tourist (also includes persons visiting family or friends)
J1: Exchange program
K: Prospective spouse
TN: NAFTA employees from Canada or Mexico
WB: Visa Waiver Program, Business; not extendable past 90 days
WT: Visa Waiver Program, Tourist; not extendable past 90 days
official website: http://travel.state.gov/content/visas/english.html