Here's an explanation by way of analogy to human beings: Person-----Woman-----Man------Baby Chicken----Hen--------Rooster---Chick A chicken may be either male or female. A hen may only be an adult female. A young female chicken is called a "pullet."
'Chicken' can refer to any bird of that species, or any adult of that species, or just an adult female of that species (a hen). It depends on the context and the speaker. 'Hen' always refers to the adult female birds.
It was also the standard term when talking to a friend of the female sex, in the NE of England, or Glasgow, in my youth. It could also be used by women addressing a man, surprisingly enough.
Hen can refer to any female bird, although it's most commonly associated with domestic birds and particularly chickens. To take one spectacular example, the peacock is a male peafowl and the female is a peahen. As gasman says, hen sometimes also refers to women - hence brides-to-be have a hen-night. One must, of course, be careful not to call the groom-to-be a cock - he has a stag-night (stag is an old word for male bird,although people now associate it with deer).
To complicate the issue, young hens are called pullets - not hens. Young roosters are called cockerels.
To summarize all your comments: Here's an explanation by way of analogy to human beings: Person-----Woman-----Man------Baby Chicken----Hen--------Rooster---Chick Young Chickens----Pullet-------Cockerel--------
To complicate matters, in BrE 'cockerel' is often used instead of 'rooster' which is more AmE. (Dictionaries say a cockerel is a young cock, but I hear the term used very generally for a male chicken.) The more traditional work is of course 'cock' (hence peacock and peahen).
