A SCRamjet, or Supersonic Combustion Ramjet, is a propulsive device with no compressor stage or turbine. Like the ramjet, it compresses incoming air via shock formation and kinetic means as a result of it's own forward speed.
The key difference between a ramjet and scramjet is that the scramjet, as it's name suggests, may combust a fuel/ air mix at supersonic velocity, whereas a ramjet slows and compresses incoming air to subsonic velocity before combustion.
The scramjet is useful in the field of hypersonic propulsion systems.
The United States is experimenting with hypersonics for military means. Air-breathing scramjets are the usual source of propulsion.