You usually see these symbols on music sheets but may not know what it means. The symbol used to signify a major 7th is maj7 while min7 stands for minor 7th. Here is an explanation of what is the difference between these two types of chords and how they are formed.
Cmaj7 = C - E - G - B
Dmaj7 = D - F# - A - C#
Emaj7 = E - G# - B - D#
Fmaj7 = F - A - C - E
Gmaj7 = G - B - D - F#
Amaj7 = A - C# - E - G#
Bmaj7 = B - D# - F# - A#
C#maj7 = C# - E# (F) - G# - B# (C)
Dbmaj7 = Db - F - Ab - C
Ebmaj7 = Eb - G - Bb - D
F#maj7 = F# - A# - C# - E# (F)
Gbmaj7 = Gb - Bb - Db - F
Abmaj7 = Ab - C - Eb - G
Bbmaj7 = Bb - D - F - A
A minor 7th chord is formed based on the major 7th chord, by lowering the 3rd and 7th note a half step (also means to flatten the 3rd and 7th). Here are the minor 7th chords in every key:
Cm7 = C - Eb - G - Bb
Dm7 = D - F - A - C
Em7 = E - G - B - D
Fm7 = F - Ab - C - Eb
Gm7 = G - Bb - D - F
Am7 = A - C - E - G
Bm7 = B - D - F# - A
C#m7 = C# - E - G# - B
Dbm7 = Db - E - Ab - B
Ebm7 = Eb - Gb - Bb - Db
F#m7 = F# - A - C# - E
Gbm7 = Gb - A - Db - E
Abm7 = Ab - B - Eb - Gb
Bbm7 = Bb - Db - F - Ab
