All engines are essentially the same thing - mechanical reactors designed to react fuel with air at as high temperatures and pressures as are mechanically feasible and then to extract useful work out of the surplus of energy created by that reaction.
Thus, All engines have essentially 3 phases... Mix/Compress, Combust, extract. (An otto-cycle motor adds an 'exhaust' stage, but in most turbine engines the simple fluid flow through the engine makes that un-necessary.
Now, the second part of the answer comes down to the properties of the air fluid in the atmosphere. Air has momentum, but not that much. Thus, at slow speeds, air flows around an object without compressing (much), at higher speeds, however, the air has more momentum than it does 'time to get out of the way', and compression starts to build on the front of the aircraft.
Obviously, this compression is going to happen any time an aircraft starts moving quickly, so you might as well take advantage of it... At very low speeds (below .3Mach), piston engines are the most efficient (despite their complexity) because there is very little compression caused by the vehicle movement. At higher subsonic speeds, conventional turbines become more efficient, because their large frontal area allows them to take advantage of the compression caused by the aircraft's forward motion... HOWEVER, the blades used in turbine designs loose drastic amounts of efficiency as soon as the flow within the goes supersonic, so most jet engine intakes are actually designed to slow the incoming stream so that the front fan always sees a subsonic stream.
As Speeds increase even higher, the amount of compression caused by the forward motion starts to become quite substantial... in the 2.0Mach range, it actually becomes feasible to use solely this forward compression motion to run an efficient engine.... This is important because the drag cost of slowing down the air to subsonic before it strikes the large front fan element starts to get prohibitively high the faster you fly- this in the 2-3Mach range, ram-jets make a lot of sense.
Scram jets are just the next iteration of ram jets... In a ram jet, while there is no longer a need to slow the incoming flow to subsonic for a large fan element, there is still a need to get the pressurized flow subsonic by the time it reaches the combustor- simply put, the combustion process cannot propagate faster than the speed of sound, thus if the internal fluid flow speed gets too great, the combustion will simply blow out... again, not an issue in the 2-3mach range, but once again at hypersonic speeds (3mach plus), the drag from the intake system slowing down the incoming air stream begins to be a considerable drain on the efficiency of the system.
Instead, scram jets take advantage of the pressure in the supersonic shock-wave of the vehicle moving forward, and use that to 'detonate' the fuel (create a region where the fuel-air mixture reaches such high temperatures and pressures that it spontaneously combusts). Essentially, in a scram-jet powered aircraft, the entire aircraft is used to provide the basic functionality of the engine. The front of the aircraft is shaped to create an ideal shockwave to compress the incoming flow, the midsection of the aircraft is shaped to provide a combustion region, and the aft of the aircraft is shaped as a nozzle to extract the energy from the combusted flow. No moving parts- the shape of the aircraft itself provides all stages of the powerplant process.
This is actually a tremendously (theoretically) efficient model... there are tons of practical issues however which make this practically very difficult... not the least of which is heat dissipation (the same system that compresses the air in the engine also compresses the airflow over the entire aircraft. Ideal gas law tells you that when all other factors are constant, compression = heat... at transonic speeds, that's a LOT of heat soaking into the aircraft that needs to go somewhere). Other issues are simple practical issues of keeping to supersonic combustion going at a wide range of mach numbers (shockwave shape varies widely with Mach speed, so you need to find a way to 'focus' all of these differently shaped shockwaves in such a way that they provide sufficient detonation temperatures and pressures throughout the speed range of the vehicles, otherwise the system will abruptly flameout). Last but not least, controlling all of these various issues means very explicitly controlling the final shape of the aircraft, and it forces configurations that are not particularly suited to slower flight (tremendously high landing speeds, etc), making overall controllability throughout the aircraft's entire envelope a problem.
But... theoretically, its a pretty cool idea.