Russian visas are issued by Russian Consulates in your country of residence, based on an invitation from a Russian party,which consists of: 1) A standard tourist confirmation from the authorized hosting Russian travel agency,which must contain the agency's reference number, registration number, official seal and signature of authorized person. 2) Voucher issued by an authorized travel agency (or confirmed hotel reservation), indicating full name of the traveler, date of birth, passport number, dates of entry and departure, name of the hosting Russian tourist company. You must submit this invitation documents to the russian consulate to obtain visa.
Within 3 days after arrival you must have your visa registered. If you stay in hotel, it will be done on default, if you you are staying with your friends, remind them that they have to help you to do that. Your russian host have only to go to the nearest post office and send filled out form to the immigration department. Sealed receipt of this mail will be your registration document.
Please note that at the moment the registration is a requirement and it is good to have it. Keep the document until you left.