most are just a little door that opens out. It is really up to you on how you build it. Here is a pic of my "new" one that I put on Coop #2 last night...
my other one just opens out like a regular door and has a ramp attached to it. If your coop is high up you may want to make it open up and have a long ramp with runners on it. There are lots of pics if you go through the posts.
Well you can either just hinge it and have to go out and open/shut every day or you can get an automatic one. I am in the process og building my first coop and plan to purchase an auto door. These are the 3 I am looking at.