When you purchase a firearm, you will fill out a 4473 form. At the same time, the Dealer is required to make a entry into his Firearms Log.
The Firearms Log is required for any dealer. The dealer must log in firearms that he takes possesion of and log them out when they leave his business. The log book stays with the dealer untill the business closes for good. The book is then required to be sent to the ATF. The book is considered a Federal Document and is treated as such.
The laws covering the Firearms Log is different than for the Form 4473.