Dangers If Some Other Person Use Your Wi-Fi Network

Some users see no harm in sharing the Wi-Fi love, giving neighbors and even total strangers free access to the Internet. Others maintain that piggybacking can open their networks to potential danger. The decision is yours, but if you choose not to encrypt, make sure to disable file sharing. Assuming you have your hardware firewall turned on, the worst that probably will happen is that your throughput will drop if your piggybacking neighbors crank up BitTorrent every evening. If you want to determine whether others are using your unsecured network, most routers have a page that lists all the wireless clients currently connected.

Two guys are working on their network like they are chatting, doing their work on their PC or Laptop but the bad guy can secretly monitor what sites you are visiting and what are you doing on your pc.
Bad guy can steal your information without you knowing.

Most people, however, are not comfortable with the idea of allowing just anyone to use their wireless service. In theory, the practice can make your network vulnerable to hackers, since anyone who uses your wireless signal is on your home network. Malevolent users, for instance, could release nasty viruses or hijack your PC. That's not terribly likely, but you should avoid the possibility by using WPA to protect yourself and keep others off your network.