Wireless Internet, also known as Wi-Fi, is the wireless transmission of Internet data between a router and a device.
The most common implementation of wireless Internet is in a home, where a central wireless hub or router broadcasts a signal. This signal is picked up by a Wi-Fi device, which communicates with the router.
Currently, wireless transmission of HD signals is still in its early stages, so you'll still need HDMI cables for that. Not all wireless products are the same. Most wireless speakers, for example, use infra-red or a technology similar to cordless phones.
For more on the transmission standard of Wireless Internet, check out our page on 802.11 and its various incarnations.