To visit Russia almost all foreign visitors need a VISA.

To apply for a Russian Tourist Visa to the Russian Consulate you need to have a visa support letter (invitation) from a Russian Tourist Company or Hotel.
A visa lists entry/exit dates, your passport number, children traveling with you, cities to be visited and visa type (Tourist visa or Business visa). It's an exit permit too, so if you lose it (or overstay), leaving the country can be harder than getting in.

You should register your visa with within 3 working days after your arrival in Russia to avoid late registration fees. You *do not need to register your visa* if you are in the country less than 3 working days. The registration has to be made by company providing your visa support letter, or Hotel.

If you don't want to stay at hotel, you can get only a "formal" invitation from some companies or apply for it to some travel company in your country to get the visa support without hotel. 

You can easily find a lot of these companies on the Internet, for instance: http://www.visatorussia.com  

If you wish to stay in one of my apartments, I also can provide you a visa support letter and register your visa in St. Petersburg. Contact me for details.