A wireless adapter is hardware device that is usually attached to a computer or other device to allow connection to a wireless network. Nowadays wireless adapters are everywhere, usually but not always as data transfer and amplification points. It is basically an amplifier for your wireless signal. Wireless Adapters serve to expand a computer's functionality such as memory cards or PC cards.
These adapters can be located internally or via an external connection. Most devices are equipped with internal WiFi. For most people who simply need to boost their signal to another location, a WiFi Adapter usually does the trick.
In the past getting an USB adapter to work with Linux servers was a problem. Now, the chipset WiFi adapters (also known as dongles) make most Adapters compatible with Linux systems and platforms.
There are a lot of choices around when it comes to these adapters. Again, nowadays most devices come standard with an installed dongle. Be sure to get a set up that meets your requirements. A check should always me made to be sure the USB adapter you are looking at is good for your particular set up. You will definitely want to be sure to get the appropriate drivers for your dongle as well if your using a PC. This will be important for PC users (MAC don't have drivers).
Not all dongles have the same throughput. This will probably the most important thing to pay attention to. For standard data transfer, 150 Mbps data transfer rate should be fine. However, if you are a gamer, or have some other high thru-put needs, you would want to have at least 500 Mbps rate of data transfer. At any rate you should be sure to get the rate of data transfer you need.
Most Adapters will tell you what systems they work well with. If you have an adapter and are making an upgrade, you want to be sure that the new adapter will be similar (compatible) in regard to specifications.
Another issue to consider is the distant between router and wireless adapter. The further you get away from the router you increasingly loose internet speed. The closer you are to there router, the stronger the signal. You will want to be sure to get an adapter that will cover the distance the signal will travel. This is important because of the many adapters on the market. They are all different.
The bottom line is to be sure you understand your current system and make sure the adapter you get is suitable for your current system.