Hacking a wireless network isn't actually that hard conceptually. Basically, the hacker just needs to get their hands on your wireless encryption key. Once they have that, they can probably just log on as if they were a legitimate user.
It seems like it's par for the course to use an analogy when covering Internet usage. Since I don't want to talk about a series of tubes, let's use the ferryman idea. When you're using wireless Internet service, your computer's wireless Internet card is communicating with a router which is then routing your requests through a modem and onto the Internet. So this is a ferry service that takes you back and forth to the big city from your remote village.
Your wireless Internet connection will ideally work just like this. You will confirm the password on your first trip to show the ferryman your ticket, he'll remember your face for awhile and in that time he'll happily take you back and forth along the route.
Of course, someone might want to leech off your service either to watch where you're going or just get their own trip for free. Note that they are bound by location. Wireless Internet becomes fairly useless after a fairly short distance. For our example, that means they have to also be in your village and close to the ferry's route. All they need to board is the password.
One method is to just guess systematically until they get it right. Of course, most modern routers will block them out fairly quickly, since the ferryman will know something is wrong if they miss it several times in a row. The password is long (usually 13 alphanumeric characters, which would equal 36^13 possible passwords), so it's a rarely effective method. Only a ferry with really poor security would wait around and let them keep guessing.
It is more likely that they'll take advantage of a weakness in the wireless Internet setup. In order to stay connected, the router sends out a periodic status report to the connected device. In this encrypted packet is the key to the router. A hacker can watch the connection, capture all of the information for awhile and then run a program to crack the encryption. The cracking program can usually pick the vital data right out for them (I have a separate article covering these wireless Internet hack programs). Once they have the password they can log onto your network.