A diatonic scale is simply a scale that uses seven tones. A major (aka Ionian), minor (aka Aeolion - natural, harmonic, or melodic), Dorian, Phrygian, Lyrdian, Mixolydian, and Locrian. Then there are blues scales and so forth. I recommend searching this site or going to cyberfret.com scales section and looking for the different scales and learn how they are different.

Lots of people are familiar with Pentatonic scales, if you are, they are the same thing only scales which use five tones.

Since you didn't really specify a type of scale ,the notes on a Am scale can be many different kinds.

Hope that helps.