When using public Wi-Fi networks (hotspots) your data is in danger of being intercepted and sniffed into by other users of the same network. This can result in leaks of sensitive private data like passwords or credit card numbers. There are even programs that allow others to take over your Facebook identity or your email account. When that happens, hackers can send mails in your name or - even worse - change the passwords of your accounts and take them over (for example online shopping or banking accounts).