それぞれEach, Respectively

Both 各 and それぞれ express the idea of 'each' or 'every', but they differ in usage. 各 always goes before a noun, for example, 各部屋 'each room', 各階 'each floor', and so on. それぞれ, on the other hand, can stand alone at the start of a sentence or be attached to a noun with の. For example, 人はそれぞれ違う意見がある ('People each have differing opinions.').

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.'

当たり and それぞれ can both convey a sense of 'each', but their nuances are different. 当たり attaches to numbers to mean 'each' in the sense of 'apiece', e.g. １個当たり１００円 ('100 yen each' / '100 yen apiece'). In contrast, それぞれ means 'each' in the sense of 'to each their own', e.g. 人はそれぞれ個性がある ('People each have unique traits').