Like most homes with more than one computer, we have wireless network access set up in our home. This makes it very convenient for me to prop the laptop onto my lap and write articles while watching the Golden Globes.
Unfortunately, anyone driving down the street can pick up my wireless connection as well. They would have to be pretty close, but neighbors (especially in apartment buildings) would have no trouble 'borrowing' a signal. This often happens on accident.
Your computer is dumb, and will connect to the strongest wireless signal it can find. If your neighbor has their wireless placed so that your laptop or computer picks up that signal stronger, then it will connect to their network. This is not a safe situation.
Your neighbor can see everything on your computer while you are connected to their network. They can also watch every bit of traffic you send across the air and then through their cables. As horrible as it may be, they might be doing something illegal, and if your computer has been using their network you may be implicated.
Borrowing signal and your laptop randomly connecting can both be overcome fairly easily though. The simplest matter is to enter your wireless router's settings and change your system identification (SSID) from 'linksys' or whatever the default name is to something you pick. This will let you be sure you connect to your home network and help your neighbors stay off of your network.
The most important step to securing your home wireless network is to enable encryption between your computer and wireless router. There are 3 types of encryption – WEP, WPA, and WPA2.
Something is better than nothing, and that's what you will get with WEP. It is extremely easy to crack within minutes, and how-to's are all across the internet to follow the process.
WPA and WPA2 are good enough to keep 99% of home wireless networks secure. Recently there have been reports of flaws in WPA, but they are theoretical and very unlikely to matter unless someone wants into your network traffic badly.
There you are, two simple steps to keep your private network private. There are many different manufacturers, and each will have slightly different steps to making these changes. A quick trip to your help menu or manufacturer's website should help you make necessary changes and keep you safely surfing from the couch.