[CentOS] repositories

Hi all :-)
This is my first post: I'm coming from debian/bsd world.

A question about repositories:

minimal installation (version 7) provides:

CentOS-Base.repo
CentOS-CR.repo
CentOS-Debuginfo.repo
CentOS-fasttrack.repo
CentOS-Sources.repo
CentOS-Vault.repo

I known there are others repositories as:

RPMForge, EPEL, REMI, ATrpms, Webtatic (and maybe also others)

so, what kind of these repositories are?

Does they repositories substitute packages from main? Or only adding new 
packages?

thanks for advices and help!

Pol