Wireless 

You can't see, hear, or touch it  .  .  .  but it is everywhere - electro-magnetic waves flowing through the atmosphere.  The invisible waves used for wireless communications are in the RF (Radio Frequency) bands of the spectrum.  There are trillions of them flowing through the air, carrying information that can be captured by highly sensitive units, called receivers.

As humans, we have trouble understanding this concept, because you can't accurately portray it in a diagram or video.  The airwaves are all analog and continuous - but can be modulated by digital signals so that they can carry data too.  We can, of course, hear low frequency audio-waves, and we can see high-frequency light-waves.  But RF waves cannot be heard or seen.

NOTE: audio waves are part of the audio spectrum, which moves at the speed of sound - not the electro-magnetic spectrum, which moves at the speed of light.

Wireless vs Wireline

Today's wireless technologies are still in their infancy, and still are not the preferred method of choice for most corporate data communications.  Simply put, wireless is not yet totally reliable - it is affected by the effects of weather and barriers (buildings, trees, hills, etc.).  For critical data, corporations still prefer wire-line communications.  These connections, like wireless, can also span hundreds or thousands of miles, using fiber-optic links with backup pairs for reliability.  But they are unaffected by physical barriers and weather.

In addition, security has always been a major problem.  Wires and optic cables are buried, or run through conduit - inaccessible to the public.  Wireless waves, on the other hand, are transmitted into the atmosphere, where anyone with proper equipment can capture them.  The "fix" is strong encryption - which until recently was not widely available, and not standardized.

Nevertheless, wireless is making tremendous strides.  Wi-Fi (Wireless Fidelity) networks and PCS telephones are now ubiquitous.  In fact, many people are choosing to do away with their home phone, replacing it with their wireless phone.  The most prevalent wireless technologies are as follows:

IEEE 802.11i  is addressing the security concerns of WiFi by describing strong encryption methods.  Cafe's offering Wi-Fi Internet access have sprung up around the world, and in many countries these cafe's are the only method of surfing the web. 

IEEE 802.15 has standardized short-range wireless devices (Bluetooth) for smart homes and remote access to numerous devices, creating small office and residential PAN's (Personal Area Networks)

IEEE 802.16 is called WiMAX (Worldwide Interoperability for Microwave Access) and is the new rage for WMAN's (Wireless Metropolitan Area Networks) using the 700 MHz allocation ranges.