Wireless routers are great aren't they? With them, you can connect desktop computers without running wires all over the house and your laptops will work upstairs, downstairs, outside or whereever you are.
But maybe you've noticed when you try to connect that there's lots of other wireless networks around. Have you ever tried to connect to one? If your neighbor has left their wireless open and unlocked, you can get on their home network and maybe browse their files. Well that also works in reverse; if you haven't set your network up right, your neighbors might be browsing through your computers.