A YubiKey is a small device that you register with a service or site that supports two-factor authentication. Two-factor authentication means that each time you log in, the service will request proof that you have your YubiKey in addition to your regular username and password. Phishing, malware, and other attack methods won’t work because they would need both your physical key and your passwords to breach your accounts.
The two-factor authentication with a YubiKey makes your login secure and keeps your information private. The YubiKey requires nothing more than a simple tap or touch. There are no drivers or special software needed. Your YubiKey can be used on multiple computers and mobile devices, and one key supports any number of your accounts. YubiKeys are nearly indestructible — just add it to your keychain along with your house and car key
YubiKey can be used on
Gmail and Google Accounts
Dropbox (using U2F)
Mac OS X