"Key" roughly, any way, is the name of the main note in a piece of music--(it is often also called the " tone center`) and, by implication, the other notes that go along with it, which, pretty much, would be the scale--

When I say main note, I mean the note that melodic phrases tend move toward, away from, and around--Some melodies will use it at the end of every phrase--while some melodies will move all around it and never quite get to it until the last possible moment--

Many of you confuse the chord with the key--understandable, since, especially if you are being modal, you can often play an entire folk melody over a chord based on the tone center, but, in order to figure out the key, it is best to consider only the melody notes, because chords can often include notes that are note even in the scale--

A lot of tunes include key changes--Sweet Georgia Brown, moves through three keys in the "A" melody alone--

If you were all classically trained, this would be a no brainer, because you would just count the sharps or flats in the key signature, and you wouldn't have to understand anything--but, since you are folk musicians, and not slaves to the written line, a little understanding is necessary--Truth is, folkies need theory, because they need to understand what is happening in order to play the music in the first place--you can play classical music without knowing any theory at all!!!!