Electrum is a desktop wallet compatible with Windows, Linux and Mac OS. This wallet was created by Thomas Voegtlin in 2011 and currently it processes about 10% of all Bitcoin transactions. It syncs fast to bitcoin network without downloading the blockchain, but offers a lower level of privacy than full wallet clients like Armory. Despite this fact, your private keys are still in secure and are never sent to the Electrum server. They stay on your computer in an encrypted form and can be kept offline by enabling watching-only mode. There is also a recovery option (passphrase) for retrieving lost wallets. Another advantage of the Electrum platform is the possibility to synchronize hardware wallets like Trezor, KeepKey, HW.1 and Ledger Nano S.