The Farm Metaphor

Having been raised in the depths of the most urban of urbanities, in a single family, front-yard, backyard neighborhood on the edge of light industry, I know nothing of living chickens or roosters for that matter. KFC, Jim Dandy, Pioneer, Church's I know. Hens and pullets are a mystery. So will somebody explain to me what is exactly up with chickens coming home to roost?

Are chickens not supposed to come home, or when they do come home are they not supposed to roost? Do chickens who come home, not come home until the cows come home? That is to say, when you send them away are they expected not to return or are they returning too early?  Is it OK for a rooster to roost? And why is a rooster not a chicken? What is roosting anyway?

Somebody throw me a bone here.