The NETWORK KEY is a special password for wireless routers.
The SSID is a name, usually broadcast to wireless computers, so you know which network to connect to. Sometimes your neighbours' networks will be picked up as well.
When you ask to join the network, the computer will ask you for the Network Key. It's usually a stream of letters and numbers and it acts like a PIN number.
People will be able to see the SSID but they cannot use your wireless connection unless they have the Network Key as well.
On my Sky router, it's written on a label on the side.
If there is no label, you may need to find it by accessing the router using a computer already attached to the router.
Generally, typing 192.168.0.1 or 192.168.1.1 in to the web address part of your browser (Internet Explorer or Firefox, etc.) will access the router but it might be different for some routers.
You need to enter a username and password, This is usually the word admin for both or admin and password respectively.
Under Wireless Settings or Wireless Set Up on the screen, there will be a box marked 'Network Key' and a stream of characters (can be in capital letters). You may be able to change this if you think someone knows the key, but it will have to be changed on every computer that originally connected to it. Like changing your PIN number or your computer password.
Hope this helps.