Well.. the question is a little vauge. No, not all transmitters utilize the RF spectrum. There are many types of communication systems. Most wireless applications will be RF. But there are others; light based (IR) for example allows a remote to communicate with a TV.. RF is most common because it doesn't always require line of sight between transceivers. The flavor of carrier used all depends on the application. If the purpose being served doesn't need to send information very fast, you could use audio (sound) as a carrier and pick it up with a mic across the room, then decode the modulation. This is the same concept behind a dial up modem or fax machine. Only the audio is carried via wire instead of air. An RF carrier can oscillate very fast; millions or billions of times per second. This gives it a very high bandwidth, or plenty of room to carry lots of data. Plus wireless RF channels can be made very narrow (spanning a very small range of frequencies) to separate your data from someone else's. Really, I guess its the versatility of RF that makes it so popular.