Hen is basically a female bird meant for laying eggs & cooking.
Chicken is a breed of hen used in rearing them in poultries for the purpose of eating them later.

Hai,

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In the US, most people think of a female chicken when they hear chicken. If you say you are eating chicken, most people assume you are eating a female chicken. As roosters aren't really used for food. Hen in terms of food is usually a "Cornish game hen".  Although Jmat gave the real answer, I am just adding what people use in day to day speech.

Ummmmmmmm, too hard ;) ; i think the difference - according to my experience- is:

An explanation by way of analogy to human beings:
Person-----Woman-----Man------Baby
Chicken----Hen--------Rooster---Chick

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"Chicken" is the name of the species.

A hen is a female chicken. A rooster is a male chicken.