It gets interesting when you take one of the seven "white-note" scales and play that same order of steps and half-steps, starting on another note.
For example, G to G. Then, instead of the G major scale (G A B C D E F# G), you have G A B C D E F natural G.
This alters the chords you need to play if you're going to accompany a song in this mode (which bears the name "Mixolydian," by the way). Instead of the usual G, C, and D7 as your three primary chords, you have to dump the D or D7, because the chord contains an F#, which is NOT in the Mixolydian mode. So what do you play in its place? F major. In fact, the chord change from G to F and back to G is the characteristic sound of songs in the Mixolydian mode.d
The Mixolydian mode is found a lot in folk music from the British Isles, and much of it has migrated into the southern mountain communities of the United States, with the English and Scots-Irish who migrated to the area.
If a song in the Mixolydian mode doesn't fit your voice in the scale starting on G, there's no law or rule of music theory that says you can't change it to, say, D. In that case, the three primary chords would be D, G, and (instead of A or A7) C. No sweat!
Another mode frequently found in folk music is the Dorian mode. This (on the "white note" scales) runs from D to D. D major goes D E F# G A B C# D. But because in Dorian mode starting on D, there is no F# or C#, the notes are both natural, the three primary chords would be Dm, G major and C major. So the Dorian mode has a distinctly minor sound, but the accompanying primary chords are one minor and two majors.
You do run into the other modes, particularly in Anglo-American folk music, but the Dorian and Mixolydian are the most frequently encountered.
HISTORICAL NOTE:
The early ballad collectors were primarily interested in the ballads as poetry rather than as songs (note the titles of many early collections, such as Bishop Percy's Reliques of Ancient English Poetry). Cecil J. Sharp was a well-grounded musician, and he was one of the first to collect the tunes as well as the text. Earlier collectors who did collect tunes would hear a source singer sing a note that was outside of a "normal" major or minor scale, assume that because this was an unschooled singer, he or she was singing a wrong note—so they wrote down what they thought the correct note should be. Sharp didn't make this assumption. He wrote down exactly what he heard. And when he analyzed the tunes later on, he made the discovery that the singers were NOT singing "wrong notes," the songs they were singing were modal!
That was a very big "AHA!" and one of the things that makes Sharp a very important collector both in England and in America.
Contrary to popular belief, modes are not that strange and mysterious, nor are they as complicated as most people try to make them.