In addition to registration of claims to copyright, the Copyright Office also records documents relating to copyright. From January 1, 1978 until February 28, 1989, if you were a transferee of an exclusive right, you had to record the transfer document in order to sue. That provision, found in then Section 205(d), was abolished (at my lobbying) in the Berne Convention Implementation legislation. The Library of Congress has, however, been recording documents related to copyright since 1870, and continues to do so today.
Documents are indexed under the names of the parties and the titles of the works listed in the documents. Once a document is recorded, a unique identifying number is assigned, while the original documents are returned. Unlike with registration, the Office does not make any determination as to the document's validity or legal effect. Its just there on the public record.
Why record, then, after March 1,1989? One reason, and it is important, is that if a registration is on file, it works to give constructive notice so that a recordation will give priority for conflicting transfers. This is a Brooklyn Bridge issue: if the Brooklyn Bridge is sold twice, who owns it? While the answer to that question is no one since it can't be sold, where a copyrighted work can be sold, it does happen that it is sold twice by the same person. And just like in real estate, the Copyright Act has a way to sort out the true owner. In the parlance of real estate recordation schemes, the Copyright Office scheme is a "race-notice" statute: the first party to record without notice of the previous transfer wins. These rules are in Section 205.
Recordation is also important in bankruptcy situations, where, under Judge Kozinski's Peregrine decision, unless the transferee has recorded first, a bankruptcy trustee will prevail merely by virtue of the bankruptcy filing. That rule is particularly harsh since a search of the Copyright Office records will not reveal the bankruptcy trustee's priority. Marvel Comics, Sony, and Paramount were tied up in litigation over this for years, delaying the release of the first Spiderman movie. The issue arises far more often and significantly than one would think.
On August 1st, the Copyright Office issued a Notice of Policy Decision clarifying a number of recordation issues, and should be consulted, along with Compendium II of Copyright Office Practices chapter 16.