A major third is four semitones above the root. A minor third is three. That one-fret difference is central to the contrast between major and minor triads.
Major is often described as bright and minor as dark, but mood also depends on rhythm, register, harmony, and how you play. These are useful starting descriptions, not fixed emotional rules.
Seconds, thirds, sixths, and sevenths can be major or minor. Unisons, fourths, fifths, and octaves belong to the perfect interval family; they can also be augmented or diminished.
Pick up your guitar
Compare C Ionian with C Aeolian using the Mode selector while Mode locked is on. Notice E becoming E♭.