Are hens and roosters the same species?
The main difference between a rooster and a hen is that the former is male and the latter is female. Their appearance differs in several ways [1].

A rooster also known as a cockerel or cock, is a male gallinaceous bird. The term usually refers to a male chicken [2]. The hen or the chicken (Gallus gallus domesticus) is a domesticated fowl, a subspecies of the Red Junglefowl [3].

And yes, the chickens laying eggs could be the same as the chickens that we use for poultry meat.

References:


Anjus Chiedozie. Rooster Vs. Hen on eHow.com
Wikipedia. Rooster
Wikipedia. Chicken