I would suggest that accidentals were passing notes to the wrong person, thus providing endless hours of embarrassment.
However.
Grace notes are when you play a note just before you play the "real" note you were aiming for. A common example is when you play a hammer-on. The note you want to ring out is the note you hammer, but you want the warlier note to sound *just* before you sound the "proper" note. In other words, the grace note doesn't sound for long at all, but is more a tiny fleck before going to the "real" note.
An acdcidental is a note played that is in a different key to the key of the song. In other words, when in the key of C major (C, D, E, F, G, A, B), you may play a Bb. That is an accidental, because the note is not part of the key signature (and not part of the key), but is played anyway.
A passing note, is often what people do with accidentals, and often sounds the same as a grace note. Basically, you use this note to get from one note to the other. If you want to move from G to F in the key of C, you might use an F# as a passing note. It sounds much the same as a grace note, it is an accidental, but it acts as a "Passing note", between the G and the F.
Hope this helps.
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