ずつOne at a time, Per, Each, Gradually

Both express the idea of 'each', but with different nuance and usage. ～ずつ attaches to quantity expressions like numbers or 少し to mean 'each' or 'at a time', e.g. 一つずつください 'Give me one of each.' or ちょっとずつよくなってきた 'It's gotten better a little at a time.' 当たり also attaches to Number + Counter, but means 'per' or 'apiece', e.g. １個当たり５００円です 'They're 500 yen apiece.'

Both convey the meaning of 'each', but differ in structure. ～ずつ attaches to quantity expressions to mean 'each' or 'little by little', e.g. 一つずつください 'Give me one of each.' or ちょっとずつよくなってきた 'It's gotten better a little at a time.' それぞれ appears before a phrase or with の and emphasizes individuality, e.g. 人はそれぞれ個性がある 'People each have unique qualities.'

Both mean 'each', but with different usage. ～ずつ follows quantity expressions to mean 'each' or 'at a time', e.g. 一つずつください 'Give me one of each.' or ちょっとずつよくなってきた 'It's gotten better a little at a time.' 各 appears before nouns and means 'every' or 'each', e.g. この電車は各駅にとまります 'This train stops at every station.'

Both deal with repetition or intervals, but in different ways. ～ずつ means 'each' or 'at a time' and attaches to quantity expressions, e.g. 一つずつください 'Give me one of each.' おきに follows time or distance intervals to mean 'every (A)', e.g. ５分おきに電車が来る 'Trains come every five minutes.'

Both express regular repetition, but in different structures. ～ずつ attaches to quantity phrases to mean 'each' or 'little by little', e.g. 一つずつください 'Give me one of each.' ごとに follows nouns and means 'every (A)', often time-based, e.g. ３０分ごとに体を動かす 'I move my body every thirty minutes.'