Here is the story on how a chicken comes to be. (essentially, the process is mostly the same for all birds, although incubation times are typically different)
A chicken lays one egg every 25 hrs....ish, and they only lay when it is light. She will lay the eggs in the same location every day until the day where "her schedule" meets the darkness. This is when she will sit on the eggs and remain on them. This pile of eggs is called a "clutch" Up to this point, she doesn't actually sit on them. For the chicks inside to actually begin forming, the eggs must reach 99.5 degrees F for 24 hrs. (this is the reason all of the eggs typically hatch at the same time.) Momma hen will remain on the eggs, with occasional quick trips to water and feed, until the eggs hatch. Chicken eggs take 21 days, but she doesn't know this. I have had some hatch in as few as 18 days, and some take as long as 23. Just before the chick hatches, it absorbs the yolk, which will allow it to stay alive for up to 3 days without food or water. They will have to stay with the hen for at least 5 weeks to regulate their body temperature by tucking into momma's feathers. Once they have their own feathers to keep warm, they are pretty much on their own and even their own mother will begin using the "pecking order" with them.