The United States requires that most individuals from around the world apply for and obtain a visa at a US Embassy in their home country prior to entry. If an individual comes to the United States without the required travel documentation, then s/he may be immediately placed in secondary inspection where s/he will be interrogated for anywhere from one to several hours. The US Immigration and Customs Enforcement (US ICE) Officer is there to determine the purpose of travel and, in some instances, whether the entrant has a credible fear of returning to his or her country of departure/origin.
Most individuals, however, will have a valid travel document or visa. A visa is obtained by making a written application, usually by submitting Form DS 156, Application for Non-Immigrant Visa, at the local US Embassy in the country of origin. The most commonly requested visa is the B-1/B-2 Tourist Visa. This visa will allow the recipient to travel to the US for up to six (6) months at a time, if the stated purpose of travel is for pleasure, or up to one (1) month at a time, if the stated purpose of travel is for business. Obtaining this visa may be easy for some and difficult for others. Usually, the consular officer will try to determine whether the applicant will likely abuse the visa and overstay or if the individual will return to his or her country of origin in a timely manner. The consular officer may determine these outcomes by using his discretion and by requesting information from the applicant such as documentation that they are returning to a home or house they own, a job, or to reunite with other close family members. Thus, it is important for the applicant to understand that s/he must convince the officer that they will most likely return to the country or origin and also that s/he will not work while in the United States. The consular officer may request documentation about the applicant's finances, such as a bank statement or tax returns to determine the self sufficiency of the applicant.
A formal letter of invitation from a family member, friend, or sponsoring organization may sometimes help with getting a tourist visa. This process not only includes a letter, but also financial documentation to show that the host will provide room and board or financial support for the payment of medical bills if the need arises while the visa applicant is traveling in the US.
I hope this helps. If this has helped, please accept so that I may continue to help others as well.