A reasonable way to achieve a long term backup of OpenPGP (GnuPG, PGP, etc) keys is to print them out on paper. Paper and ink have amazingly long retention qualities – far longer than the magnetic or optical means that are generally used to back up computer data.
Due to metadata and redundancy, OpenPGP secret keys are significantly larger than just the “secret bits”. In fact, the secret key contains a complete copy of the public key. Since the public key generally doesn’t need to be escrowed (most people have many copies of it on various keyservers, web pages, etc), only extracting the secret parts can be a real advantage.
Paperkey extracts just those secret bytes and prints them. To reconstruct, you re-enter those bytes (whether by hand or via OCR) and paperkey can use them to transform your existing public key into a secret key.