What is the difference between nginx config files? [closed]

i have noticed that nginx has a number of configuration files such as nginx.conf in the /etc/nginx directory and 2 others one in the /etc/nginx/site-availabe called defualt and one in the /etc/nginx/site-enabled called also defulat. Does anyone knows what is the difference between all those?

Thanks in advanced!

In ubuntu by defualt the site config is from /etc/site-available i believe.