What does it mean when a piece of music is in a certain key?

No, when a piece of music is in a certain key, it means that that note/chord is the tonal center. For example, in a key of A minor, it is usually the A minor chord which is the "goal" of the song. It usually ends in an A minor chord. And everything else like the melody, intervals, etc., revolves around this "progression towards the A minor chord." Of course, the composer has all the freedom to include notes that are not part of the scale, but the most-used notes are the notes of the scale. The key refers to the basis wherein the song is composed. It is the song's center.

Sounds like you need to visit www.musictheory.net. Bet that will answer this and a whole lot of other questions that you probably have.

The first answerer didn't really help. =/ The difference between Minor and Major is that Minor does in fact have a sadder, more tragic feeling....but the real physical difference is that major is usually played on the white keys OR a half step above (sharps). Minor scale uses flats instead (a half stop below). If a song is in A Minor, then yes it will only use the A minor scale and nothing else along with the flats. I don't know how many flats is in the A minor scale, I would have to look it up. BUT there can be "accidentals" where instead of a flat, the composer makes a note natural (white key) or they make it into a sharp (half stop above the natural note). So it is not a dominate rule that it ONLY uses the A minor scale, it's supposed to but some songs may change a note temporarily called an accidental...but the song is still written in A minor. If you're not a piano player, you may not know what I am talking about and I don't know any other way to explain it. Good question though. EDIT: wow I got 3 thumbs down from these losers. I've played piano for 7 years and I'm still taking lessons, I think I would know! You people in this section are just elitist pricks and I gave an answer prior to what I know...

it means that the peice is played in a up toen or down tone. like minor sounds unhappy and major sounds happy and that tone of the music is kept all the way through it.