Use the adjective each when every separate person or thing in a group is considered individually, one by one or piece by piece. If all the guests at your party will go home with a goody bag, you can say that each person will get one.
The word each is useful for singling out one instance, item, or person from a larger group. When it's used as an adverb, it basically means "apiece." You might, for example, say, "These cupcakes cost me three dollars each," or "My siblings and I have one cat each — all together, we have six cats in our family." The Old English root of each, , is based on a Germanic phrase that means "ever alike."