|I assume you're connected wirelessly.|
I furthermore assume your wireless router is not encrypted and not MAC address filters are set.
So your wireless router is open for everyone in your environment. Everyone can connect to it, gets a valid ip address and can see all computers connected to your network.
This is a big problem, but can be solved by configuring the wireless router to use encryption (WPA, WPA2, WEP) and configure MAC address filter, so that only your computers are allowed to connect to your router wirelessly.