Electrum is a feature-rich but not very user-friendly Bitcoin desktop wallet. If you're just starting out, I recommend avoiding Electrum, but if you already know the basic terms, it's a great choice.
Created in 2011 by Thomas Voegtlin, Electrum has since been modified by different developers (the code is open source). Electrum is a client wallet - which means that to use it you have to download and install it on your computer. The client is only available for the office, which is a major drawback for users who want to use their mobile as a wallet.
Electrum is proud to be fast, safe and private. Your private keys are encrypted and never leave your computer. Your wallet can be retrieved from a passphrase (also known as a source phrase). Your private key is also not shared with the server connecting Electrum to the Bitcoin network. The server does not store user accounts.