Each major key has a special relationship with a particular minor key and vice versa. When the two keys share the same key signature but a different tonic we say that one key is the relative minor (or relative major) of the other.
For example, G major and E minor both have a single sharp in their key signature; so we say that E minor is the relative minor of G major. The relative minor of a major key always has a tonic a minor third lower.
The only difference between the two relative keys is which note you start with. The minor scale starts from the sixth note of the major scale. Don't think that this is small difference. It changes the tonic so all the chords etc.. For example;
Major Scale: G - A - B - C - D - E - F# (1, 2, 3, 4, 5, 6, 7)
Relative Minor Scale: E - F# - G - A - B - C - D (1, 2, b3, 4, 5, b6, b7)
You can find all the relative major and minor keys below;
|C sharp/D flat major
||A sharp/B flat minor
|D sharp/E flat major
||C sharp/D flat minor
|F sharp/G flat major
||D sharp/E flat minor
|G sharp/A flat major
||F sharp/G flat minor
|A sharp/B flat major
|B/C flat major
||G sharp/A flat minor
Parallel major and parallel minor are major and minor keys that share the same name, but use different key signatures.
Easily calculating the key signature of the parallel major or minor of a key
If you add 3 flats to the key signature, you will find the parallel minor of a key
For example, F major has 1 flat (B). Adding 3 flats would yield 4 flats, meaning F minor has 4 flats in its key signature. (B E A D)
B major has 5 sharps (F C G D A). To find B minor, add 3 flats. Since flats cancel out sharps, you are left with 2 sharps. (F C)
To find the parallel major, add 3 sharps.
Em to EM: Em has 1 sharp (F). Add 3 to get 4 sharps. (F C G D)
Fm to FM: Fm has 4 flats (B E A D). Add 3 sharps to get 1 flat. (B)