Everyone know that Russian visa is... kmmm, quite difficult to obtain. Why? Actually, I don't know. Sounds strange, because a lot of people from nearby countries (not so safe as Europe and USA, for example) don't need visa... And as a first step to obtain it, you need to apply for a visa invitation. Once you have purchased your visa invitation (and obtain OTHER documents) you can apply for your Russian Visa... These are not simple letters from inviting you, but an official document corresponding to canons of ex-soviet bureaucracy. Invitations can only be issued by accredited organisations and need to be purchased. Hopefully one day in the future they will be free!.
Important: you do not have to obtain the invitation from every accommodation and every city you plan to stay. You only need one invitation for your whole stay in Russia.