Steel guitar, strictly speaking, refers to a method of playing using a metal slide (or 'steel') on a guitar played horizontally, with the strings uppermost. This covers lap steel guitar, pedal steel guitar and 'Dobro' style guitars. The technique was invented in and popularized in Hawaii, thus the term Hawaiian guitar or as it known in Greek, havaya (χαβάγια), more common in the early 1900s; and is predated by bottleneck guitar.
A 'Steel Guitar' is one designed exclusively to be played using a steel slide. Most commonly, these are the lap steel and pedal steel. The lap steel is a simple instrument, typically having 6 strings and tuned to either standard tuning, or an open chord. The pedal steel can have between 9 and 13 strings, and sometimes two or even three necks, each in a different tuning. Pedals and knee-levers are used to alter the tunings of different strings, which gives the instrument its distinctive voice, most often heard in popular (laika) music.