According to Russian law for getting a Russian tourist or business visa you must have an invitation letter.
Tourist invitation consists of two parts – the tourist confirmation letter (“подтверждение о приеме иностранного туриста” – in Russian) and the travel (tourist) voucher – in this documents there is necessary information which needs to apply the application for a Russian tourist visa (name of travel organization, its address, reference and confirmation numbers).
Business invitation from Russian Organization is a document in the Russian language – in this document, there is necessary information which needs to apply the application for the Russian tourist visa (name of travel organization, its address and TIN number).