Raspberry Pi Hotspot (Access Point)
See the official Raspberry Pi Wireless Access Point Guide.
Note that the integrated WiFi chips built in to certain Raspberry Pis allow them to work as access points.
Not all USB WiFI adapters can be used to broadcast WiFi! Do not work under the assumption that you can buy any USB WiFi adapter from Amazon (or wherever) and use it for a WiFi AP.
Feel free to contact me with questions or feedback regarding this article.