Strictly speaking, a steel guitar is any guitar made out of metal, which means it can be a resonator guitar used for the blues and Americana music, a lap steel guitar popular with those who play Hawaiian music, or a venerable pedal steel, which is what gives country-western music and Texas swing their distinctive sounds.
While the first resonator guitars made in the 1920s only featured an aluminum amplifying cone where the guitar's sound hole would be, subsequent resonators had all-aluminum bodies. Resonators grew out of the popularity of the acoustic guitar in the 1920s—the amplification helped them be heard over louder instruments such as banjos.
The invention of the resonator is widely credited to John Dopyera who, in the mid-1920s, developed an amplifying system for ukuleles. He founded the National String Instrument Corporation in August 1926 with his brothers Rudy, Robert, Louis, and Emil, and their company would use John’s system to produce “ampliphonic” or “self-amplifying” guitars, known today as resonators. But Dopyera was frustrated with the sound his uke-sized resonator produced when fitted on guitars so, in 1929, he left National and founded Dobro Manufacturing.
Lap-played steel guitars produced by Gibson, Epiphone, and National were produced in the 1930s to cash in on the craze at the time for Hawaiian music. Fender and Gretsch also produced lap-steel guitars (some of which were misnomers since they had stands), but not until the 1940s. Because these guitars lacked sound holes for natural amplification, even the earliest models were electric—Gibsons like the E-150 and Roy Smeck Special, both of which had Charlie Christian pickups, are especially prized by collectors today.
Finally there are pedal steel guitars, single neck or double, which are mounted on stands and fitted with anywhere from four to 10 pedals, each of which can change the pitch of selected strings. Vintage Fender steel guitars, with or without pedals, are favorites of players and collectors alike, as are Sho-Bud instruments—the fingerboards of Sho-Buds are known for their inlay of diamonds, spades, hearts and clubs.
Regardless of the type of steel guitar played, one thing they all have in common is that the player needs to be adept at finger picking—these instruments are not meant to be strumed. In addition, instead of using the non-picking hand to push down the strings to produce chords, the player holds a tube—sometimes made of steel, sometimes glass—lightly against the strings. The tube is slid up and down the fingerboard, which is why the technique is also known as slide guitar.