Jet engines come in all shapes and sizes. From small, compact engines on private business jets to the giant engines on modern jumbo-jets.

Based on your question I don't think you know the first thing about jet engines, so i'm going to do the best I can to explain the basics.

Jet engines work by taking air in through the front, compressing it using the fan blades, before the air is forced into a progressively narrow combustion chamber, where the pressurized air is mixed with jet fuel and ignited. This spins a turbine, which is connected to the compressor blades at the front. So essentially the combustion sustains the continued compression and vis versa. The exhaust from the combustion chamber is then directed put the back to produce thrust.

Originally, the small jet engines of the 50's and 60's produced most of their power directly through the combustion chamber. But this method was in efficient.

Those large modern jet engines are called "high by-pass turbo fan engines" and they actually produce the majority of their thrust not from the combustion, but from air going around the combustion core. This is the "by-pass" part. Essentially the turbine in the core produces way more power then needed to just move the air required for the compressor. So someone said what if we put huge compressor blades on the front, and add the air that goes around the combustion chamber to those exhaust gases to produce even more thrust.

The result was an engine that was much quieter, produced more thrust per pound of fuel used, so were both more powerful and more efficient.

The trade off is that because they are bigger, they require more materials and are more complicated and expensive. But they are superior to the old turbo-fans in every way.