It's a good question, because it's more complicated than it seems. The word 'chicken' has two main meanings: 1) As an uncountable noun, it's the food we eat. e.g. We had chicken and rice for dinner. 2) As a countable noun, it's a general word for the whole family of birds e.g. My grandmother has a lot of chickens on her farm. A hen is the adult female bird. A cock, cockerel or rooster is the adult male bird. A chick is the baby bird. By the way, the words 'hen', 'cock' and 'chick' can also be used to refer to any species of birds (ducks, swans, eagles, penguins, parrots..... all birds!). They are simply the general terms for female, male and baby birds. These are also the specific words for the female, male and baby of the common domestic fowl that arrives on our plates as 'chicken'. I did say it was complicated...

chicken is a species of bird. Hen is a female chicken (rooster is male)