Workgroup
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A Workgroup is a software identifier that PC's use. Every Windows PC has a Workgroup name. If you want two PC's to be on the same network, you set the workgroup name to the same value. By default, most Windows PC's have MSHOME as the default workgroup. You can change this to a more appropriate identifier, such as your name, HOME-NETWORK, etc. Just remember to set it to the same value on all PC's.
NetBIOS Name (Computer Name)
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The computer's individual identifier, it is how you find other computers on the network. For example, "ADAM-LAPTOP" is the name I set for my laptop (I'm Adam, I have a laptop). I set my desktop to "ADAM-DESKTOP" and when I want to connect to my desktop, I just type in //ADAM-DESKTOP from my laptop and I can access files on my desktop. Individual PC's are grouped under a workgroup (think of a workgroup as a folder, with all the PC's in the workgroup like files in that folder, that's how Windows groups PC's).
LAN
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Local Area Network - it's the set of physical connections that link your computers. It can be wired (Ethernet), it can be wireless, it can have routers and switches, and it can contain a combination of all of these devices. As long as you have a single router at the top of your network, computers can all see each other even if some are connected wirelessly while others are plugged in with wires.
SSID
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This is the name of your WIRELESS CONNECTION. It doesn't have anything to do with workgroups or computer names, it is simply the name that your router broadcasts to help you locate your wireless network. Your computer will see SSID's from all wireless access points around it, you pick the one that is your connection (you set the router's SSID in its settings page) and the computer will establish a connection with the router. This wireless connection, once established, behaves pretty much like a wired Ethernet connection, connecting to the router via its SSID is the equivalent of plugging in an Ethernet cord between the PC and the router.