A wireless dongle is a small device that converts WiFi signals to a USB signal.
Adapter and dongle are often used interchangeably to describe a small wireless electronic device.
For example a wireless mouse that has its own USB adapter that transmits wirelessly to the mouse is often called a dongle.
Most often though when the word wireless dongle is used it is meant to mean a WiFi device.
These small dongles can connect any computer that has a USB port into receiving a WiFi signal and be able to connect to a network and get online.
This is useful for a variety of reasons from a computer that has no wireless built-in to a laptop that has a broken wireless card, or simply need upgrading.
Wireless dongles use the same WiFi protocols as a smartphone, tablet, or router making them useful in getting a computer on a network.
The protocols used are backward compatible as new ones are introduced.
WiFi protocols all start with the number 802.11 which is then followed by a number a/b/g/n/ac to indicate the version.
For example the newest protocol 802.11ac is also the fastest and works in the 5 GHz wireless frequency.
Some dongles are small and don’t have good range since the built-in antenna is smaller.
These are still widely used since they are very small and out of the way. These type of small adapters have about the same range as a laptops built-in wireless card.
Other dongles have large external antennas that give them better distance and are often used to upgrade a laptops poorly built wireless card or simply when a WiFi signal needs to reach father.
The words dongle and adapter can be used interchangeably. So if the word dongle is confusing simply replace it with adapter.
Even though a wireless dongle usually means a WiFi adapter it can also be used in other context such as a Amazon Fire stick or a computer mouse transmitter.
Read Here For A Guide To Buying a Wireless Dongle.