Electrum has been around since 2011 and works with Windows, Mac and Linux. It’s one of the most popular ‘thin’ wallet clients, in that instead of downloading the entire Bitcoin blockchain, it connects securely to other servers to verify your BTC balance and process payments. This means you can set it up in minutes and it takes up very little space on your hard drive.
Electrum uses a ‘hierarchical deterministic wallet’, in that when you first launch the program it generates a random ‘seed’ of 12 dictionary words, from which it derives the keys necessary to spend and receive BTC. Electrum displays the seed as you create your wallet and requires you to write it down. This means that if you lose access to this version of Electrum, you can easily reinstall it on another machine and use the seed to restore your BTC.