sources.list file is Package resource list for APT.The package resource list is used to locate archives of the package distribution system in use on the system. .
APT is a package tool program which is support the system administrator with the installation and administration of the programs.First one need to know is it's configuration file is located in /etc/apt/sources.list.APT supports a large nuber of different sources of installation.Most used sources are cdrom,file.http,ftp each of these sources are discribed in an individual file and this file is also consider the sequence of these entries from starting to ending of the file.