Hi, please what is RHEL (Red Hat Enterprise Linux), what belongs to this group?

What is then EL, EL5 and EL6

Does CentOS 5.x belongs to any of these groups?
What about Debian ?

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There is quite nice timeline of linux distributions: http://linuxhere.files.wordpress.com...ntimeline1.png
Debian>Ubuntu,
Slackware>Suse,Gentoo
Redhat>CentOS,Fedora,Mandrake

so the RHEL are linux distributions based on Redhat (centos, fedora, mandrake) for example?