Debian sources.list file contents
Ref:
http://www.mayin.org/ajayshah/COMPUTING/debian-principles.html
The sources.list for using stable reads —
# See sources.list(5) for more information
deb http://http.us.debian.org/debian stable main contrib non-free
deb http://non-us.debian.org/debian-non-US stable/non-US main contrib non-free
deb http://security.debian.org stable/updates main contrib non-free
security.debian.org is only relevant for stable since the core stable is supposed to be constant. With the other distributions, security updates are just normal package updates.
As the comment points out, you can say man 5 sources.list to learn the file format.