Take a look at images of an early model Boeing 707 and the engines are tiny compared with those on a Boeing 737-800, which carries about the same passenger load, and the reason is not the number of engines on each of these aircraft. Developed in the 1950s, the 707 was initially fitted with turbojet engines.
In a turbojet, all the air that enters the engine passes through its entire length where it is mixed with fuel and ignited to produce thrust that propels the aircraft. In a turbofan engine, a greater volume of the air that enters the engine bypasses the core engine where combustion takes place.
This bypass air is ducted around the core where it mixes with the exhaust gases following combustion. Turbofan engines require a much larger intake than a turbojet engine, and therefore the larger engine size. Turbofan engines have several advantages over turbojets, including better fuel efficiency, more thrust and greatly reduced noise.
The last commercial airliner powered by turbojet engines? Concorde.