First the download link;

https://wiki.centos.org/Download

Definition:

CentOS (abbreviated from Community Enterprise Operating System) is a Linux distribution that attempts to provide a free, enterprise-class, community-supported computing platform which aims to be functionally compatible with its upstream source, Red Hat Enterprise Linux (RHEL)

CentOS is available free of charge. Technical support is primarily provided by the community via official mailing lists, web forums, and chat rooms.

It means there is no subscription needed for the yum repos, updates etc.

Centos is very suitable for test systems,  but for mission critical applications I strongly suggest a OS with support