Short answers, pulled from the story.
A steam engine is a heat engine that performs mechanical work using steam as its working fluid. It uses the force of steam pressure to push a piston back and forth inside a cylinder, and a connecting rod and crank convert this motion into rotational force. It is an external combustion engine, meaning the working fluid is kept separate from the combustion products.
Thomas Newcomen invented the atmospheric engine around 1712, the first commercially successful engine that could transmit continuous power to a machine. The first commercial steam-powered device overall was a water pump developed in 1698 by Thomas Savery. Thomas Savery is considered the inventor of the first commercially used steam-powered device.
James Watt added a separate condenser, removing spent steam to a separate vessel for condensation, which greatly improved the work obtained per unit of fuel. He developed this improved version of Newcomen's engine between 1763 and 1775. Boulton and Watt's early engines used half as much coal as Smeaton's improved Newcomen design.
The world's first railway journey took place on the 21st of February 1804, when Richard Trevithick's steam locomotive hauled 10 tonnes of iron, 70 passengers, and five wagons. It ran along the tramway from the Pen-y-darren ironworks near Merthyr Tydfil to Abercynon in south Wales.
Advances in electric motors and internal combustion engines led to the gradual replacement of reciprocating steam engines in commercial use. Steam turbines replaced reciprocating engines in power generation due to lower cost, higher operating speed, and higher efficiency. Merchant shipping turned increasingly to diesel engines, and warships to the steam turbine.
The Rankine cycle is the fundamental thermodynamic basis of the steam engine, named after the Scottish polymath William John Macquorn Rankine. It uses the phase change of water, boiling to produce steam and condensing exhaust steam back to liquid, to provide heat-to-power conversion. In the 1990s, Rankine steam cycles generated about 90 percent of all electric power used worldwide.