A turbine is a type of engine that can extract energy from a fluid, such as water, steam, air, or combustion gases. It has a series of blades, typically made of steel but sometimes ceramic, that can withstand higher temperatures. The fluid goes in one end, pushing the blades and causing them to spin, then gets ejected out the other end. The fluid leaves the engine with less energy than it had going in — a portion of the difference is captured by the turbine.
Turbines are at the core of civilization, since practically every form of electric power is generated by them. When people say coal power, nuclear power, hydrothermal power, etc., they are referring to using some energy source to agitate a gas that then drives the blades and generates power. It is one of the most common types of engines, where an engine is defined simply as something that takes an input and generates an output. Along with heat engines and motors, turbines make up the vast majority of dynamic machinery.
Gas turbines are one of the most flexible type, and they are used to power a variety of mobile machines above a certain size, jets being the most famous application. Even the Space Shuttle uses one to combine fuel at tremendous rates. Because they can spin at extreme rates, gas turbines allow a huge amount of power to be packed in a relatively tiny space. A typical engine of this type operates between 3,000 and 10,000 rpm, and smaller variants can climb above 100,000 rpm. A recently constructed matchbox-sized one spins at 500,000 rpm and generates 100 watts. Scientists want to push them to operate at a million rpm or above, but making this possible without melting the assembly can be tricky.
To extract the greatest efficiency from turbines, they are often chained together. This can lead to efficiency levels of 60% or higher, quite amazing in the world of thermodynamics. Understanding the operation of these engines in detail is the province of mechanical engineers, though anyone who works with machines is probably familiar with the basics.