Most nuts you know are not 'true' nuts, but either seeds, legumes or drupes. Here is how you can differentiate between them:
True or botanical nuts: Seeds within hardened outer shells that need to be cracked open. Eg: Hazelnuts, walnuts, acorns.
Seeds: Edible seeds are also considered as nuts. Eg: Brazil nut, macadamia nut.
Legumes: Pods with many seeds that open out when ready to be harvested. Eg: Peanut.
Drupes: Fleshy fruits which contain a hard shell housing a seed within. In this case, we eat the seed of the drupe instead of the fleshy fruit. Eg: Pistachio, almond, cashew.
But, we commonly refer to all of them as ‘nuts’!