IN a normal jet engine air is compressed by a compressor and then ignited by fuel and shot out the back creating thrust.The ram jet, by flying through air at high speeds, forces air into the engine were it can be compressed, fuel injected, and blown out the back. Because of the fact that it can't produce compression without moving the ram-jet must either already be flying, or have it be flown on a rocket. The ability to take off is pretty majorso this fact may make the ram jet seem crappy compared to the turbo-jet.It has one enormous advantage, no moving parts. This means that there is no limit on how fast it can go. The only limit is the human body and the limitations of the parts.

http://www.aviation-history.com/engines/ramjet.htm