It's a 49.860 MHz crystal. Unfortunately I cannot tell you what the load capacitance, accuracy (Parts Per Million) and ESR are. You also need to know which harmonic it operates at. That's the problem with crystals. You can buy them at Mouser.com, Digikey and most online shops. I guess that this one is used in a transmitter or receiver.
Crystals are used as they are quite accurate. In a transmitter the crystal is often used in modern circuitry to drive a Phase Locked Loop (PLL). This is a kind of multiplier used to "up" the frequency. To answer your question, the easiest method of transmitting is using On Off Keying (OOK). Basically once you have defined your RF operating frequency you simply transmit bursts of the carrier at specified periods. One period for a logic 1 and another for a logic 0. Problem with this method is that it is very noisy and has many harmonics. FSK (Frequency Shift Keying) is another method. You have your fundamental frequency - say 868 MHz. By "pulling" the frequency in one direction by a few KHz you transmit a logic 1 and by "pulling" the frequency in the opposite direction you transmit a logic 0. So in essence your (center frequency + deviation) is one state and (center frequency - deviation) the other. It becomes interesting when trying to extract the data on the receiving side. There are other methods of transmission but they are more complex.