You need what's called a wireless adapter to connect to WiFi networks. Virtually all laptops/notebooks come with built-in "WiFi" adapters these days, but you can easily add an external adapter via USB.
A laptop spec that might read: "Internally installed Wi-Fi: (802.11 a/b/g/n Draft 2.0)," meaning it can connect with 802.11a, 802.11b, 802.11c, & 802.11d networks. In other words, all the current 802.11 standards. From the spec, you see that the adapter is also internal. Duh! :-)
Your adapter must be compatible with the same WiFi standard that the wireless network you want to connect with is using. 802.11g is currently most common, but most 802.11g routers and adapters also accomodate 802.11b.Besides internal devices, adapters also are available as plugin notebook cards, internal PCI expansion cards, and external USB devices. Most "WiFi Enabled" devices have internal adapters.
Unlike routers, adapters need drivers, which you will probably need to install. Make sure the adapter you buy will work with your PC and operating system, e.g., Windows. Mixing and matching adapters and routers from different manufacturers usually isnít a problem, but there's less chance of trouble or performance kinks with devices from the same manufacturer. And of course you'll need to install the driver (software) to use your adapter.
Wireless networks, WiFi networks, access points, or just WiFi generally mean the same thing from the point of view of your adapter. Adapters (and the software that works with them) will sense any networks (hotspots) that are within range.
It's your job to select the one you want, and then set up the connection. Security should be your number one concern, and getting connected your second. Most people get that backwards.:-)
The first two icons in this screenshot show what I see in my system tray area when I have a USB Wifi adapter installed on my Windows XP computer. The first icon (red x) shows that the adapter is installed, but there is no network connection yet. The second is the "Safely Remove Hardware" icon. Before you remove the adapter, you should click that icon, and then click the "Safely remove... (device description" message that pops up.
If you click on the the WiFi connection icon it brings up the Wireless Network Connection dialog box. Clicking the "GreenCarpet" network exposes the connect button. Clicking the "Connect" button opens the Network key dialog, where you must enter the correct key to be able to connect. If you're successful the dialog box will show that you are connected to the network.
You can also open the dialog box from the Start menu. If you have a "Network Connections" menu item, just click it to get to the wireless connection dialog. You can also get to "Network Connections" via the Control Panel.
Later on, you can open the dialog box again when you want to disconnect from the network. Your computer saves the Network Key if you are successful in connecting. If you do not disconnect and just walk away, Windows will automatically connect to this same network the next time you are in range. Otherwise, you'll need to manually connect again, but you won't need the key again.
Windows Secrets has some good information on troubleshooting WiFi connections.