Locomotive powered by steam, usually by reciprocating rods connecting steam cylinders to driving wheels. Water in locomotive boiler is converted to steam by a fire (generated by coal, oil, wood, or other source).
A locomotive powered by a steam engine. This has been the most common form of propulsion for most of the time that railways have existed.
A steam locomotive is a locomotive powered by steam. The term usually refers to its use on railways, but can also refer to a "road locomotive" such as a traction engine or steamroller.