Description of /apt/etc/sources.list

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deb http://ftp.de.debian.org/debian/ lenny main non-free contrib
deb-src http://ftp.de.debian.org/debian/ lenny main non-free contrib
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deb/deb-src, indicates the type of archive: whether it contains binary packages (deb) or source packages (deb-src)
http://ftp.de.debian.org/debian/: repository of packages, it can be ftp, http, file and so on.
lenny: distribution, e.g. stable, testing sid...
main non-free contri: different licence sections in the package repository.

• main: the official Debian distribution, full free use and redistribution.
• contri: Packages in this area are freely licensed by the copyright holder but depend on other software that is not free.
• non-free:  Packages in this area have some onerous license condition restricting use or redistribution of the software.