Liquid Fuel engines are currently the most commonly used engine type in Kerbal Space Program. They often have a high Thrust-to-weight ratio (TWR) and are very efficient, capable of burning with high thrust for a very long time. When 0.18 is released, liquid engines use two fuel types; liquid-fuel, and oxidizer, making them incompatible with jet fuel.
Liquid fuel engines make the basis of almost any good rocket/spaceplane. They give longevity, and power, and are also throttle-able, making them versatile in any environment.
There are currently seven liquid fuel engines, but there will be more added in 0.18.