Transit C-1 Visa

The C-1 Transit visa is for foreign persons travelling in continuous transit through the US on the way to another country. C-1 visas are stipulated to be for ‘immediate and continuous’ transit through the US, which includes a layover in the US on the way to your final destination but there are no other privileges.
Citizens of a country from the Visa Waiver Program may be able to obtain a visitor visa (ESTA) rather than a C-1 visa. Citizens of Canada or Bermuda are not required to obtain the C-1 visa for transit through the US.
Transit C-1 visas cannot be used for visiting friends or sightseeing and is not applicable for a crew member of a private yacht departing from a foreign port that will be on US waters for longer than 29 days.
An example of where a C-1 visa might be used is for changing airports in the US, where you will not be entering the US for tourist purposes, but are just transiting through the country route to another destination.

C1/D visa

The C1/D visa is a combined visa for crewmembers that are non-immigrants travelling through the US to join a vessel or aircraft that they will be working on.

How to apply for C-1 visa

Applicants will need to apply for their non-immigrant visa through their country’s US Embassy, where they will be required to bring documents and attend an interview.
The first step is to complete the online DS-160 form which includes personal information, passport details, your travel plans, any previous travel to the US, a contact for in the US and your work/education/training background. There is a large section covering security and background that requires a lot of information to ascertain whether you fit the eligibility criteria for entering the US under the border security controls.
Once you have submitted the DS-160 form you will need to arrange an interview at the US Embassy to attend the interview where further questions will be asked in order to determine whether your visa application is accepted.