A visa sponsorship letter is written by the person or organization a foreign visitor is staying with while in a given country. It usually includes a variety of personal information about the visitor that can be used to identify him, as well as a written statement that the visitor has been invited to the country. Different countries require different information in these letters.
According to the U.S. Department of State, sponsorship letters are not required for the majority of visits to the country. However, some visa applicants may be required to submit additional paperwork, including evidence that they are related to the people they plan to stay with, or a letter from a friend or family member that attests they are helping to pay for the trip. Other countries, like Canada, require a sponsorship letter for permanent immigrants. According to the Government of Canada, this letter must include personal information about the immigrant, as well as information about the sponsor.
Some countries require a letter of invitation for tourists as well as immigrants. According to Real Russia, anyone who is traveling to Russia needs a letter of invitation confirming his identity and the duration of his stay before he can enter the country, even if this letter comes only from the hotel where he is staying.