Fedora
Fedora(formerly Fedora Core), is Free and Open Source Software, an operating system based on the Linux kernel, developed by the community-supported Fedora Project and owned by Red Hat. Fedora contains software distributed under a free and open source license and aims to be on the leading edge of such technologies. Linus Torvalds, author of the Linux kernel, uses Fedora on all his computers.

Fedora has a reputation for focusing on innovation, integrating new technologies early on and working closely with upstream Linux communities. Making changes upstream instead of specifically in Fedora ensures that the changes are available to all Linux distributions. 

Fedora has a relatively short life cycle: version X is supported only until 1 month after version X+2 and with approximately 6 months between versions, this means that a version of Fedora is supported for approximately 13 months. This promotes leading-edge software because it frees developers from some backward compatibility restraints, but it also makes Fedora a poor choice for product development, which usually requires long-term vendor-support. Fedora users can upgrade from version to version using FedUp. 

The default desktop in Fedora is the GNOME desktop environment and the default interface is the GNOME Shell. Other desktop environments, including KDE, Xfce, LXDE, MATE and Cinnamon, are available and can be installed.

Fedora uses the RPM package management system.

Security is also important in Fedora with one specific security feature being Security-Enhanced Linux, which implements a variety of security policies, including mandatory access controls, and which Fedora adopted early on. [From Wikipedia]

Office website: http://fedoraproject.org/
Fedora Forum: www.fedoraforum.org

RHEL/Red Hat Enterprise Linux
Red Hat Enterprise Linux (RHEL) is a Linux distribution developed by Red Hat and targeted toward the commercial market. Red Hat Enterprise Linux is released in server versions for x86, x86-64, Itanium, PowerPC and IBM System z, and desktop versions for x86 and x86-64. All of Red Hat's official support and training and the Red Hat Certification Program centers around the Red Hat Enterprise Linux platform. Red Hat Enterprise Linux is often abbreviated to RHEL, although this is not an official designation.

The first version of Red Hat Enterprise Linux to bear the name originally came onto the market as "Red Hat Linux Advanced Server". In 2003 Red Hat rebranded Red Hat Linux Advanced Server to "Red Hat Enterprise Linux AS", and added two more variants, Red Hat Enterprise Linux ES and Red Hat Enterprise Linux WS.

Red Hat uses strict trademark rules to restrict free re-distribution of their officially supported versions of Red Hat Enterprise Linux, but still freely provides its source code. Third-party derivatives can be built and redistributed by stripping away non-free components like Red Hat's trademarks, including community-supported distributions like CentOS and Scientific Linux, and commercial forks like Oracle Linux, which aim to offer 100% binary compatibility with Red Hat Enterprise Linux. [From Wikipedia]

Office website: www.redhat.com

CentOS/Community enterprise Operating System
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 100% binary compatible with its upstream source, Red Hat Enterprise Linux (RHEL). In January 2014, it was announced that CentOS was officially joining forces with Red Hat while staying independent from RHEL, under a new CentOS Governing Board.

The first CentOS release in May 2004, numbered as CentOS version 2, was forked from RHEL version 2.1AS. As of versions 5.10 and 6.5, CentOS officially supports x86-64 and x86 architectures (with Physical Address Extension (PAE) required for the latter), while a beta release is expected to be available for the PowerPC architecture.  [From Wikipedia]

Office website: www.centos.org

Here’s a simple visual and description:
Fedora
-Run by Redhat(company)
-Community driven
-Focused on Quick release(~6 months)
-Stress features and functionality
-Free

RedHat
-Based on Fedora
-Run by Redhat(company)
-Released corporately by Redhat
-Focus on long release for stability
-Stresses stability over features
-Commercial(non-free)

CentOS
-Based on Commercial releases of Redhat(disto)
-Run by community
-Basically Redhat without the cost or support

They go in order starting from the top, so:
1. Fedora is the main project, and it’s a communitity-based, free distro focused on quick releases of new features and functionality.
2. Redhat is the corporate version based on the progress of that project, and it has slower releases, comes with support, and isn’t free.
3. CentOS is basically the community version of Redhat. So it’s pretty much identical, but it is free and support comes from the community as opposed to Redhat itself.

Other linux based releases OS, like Oracle Linux is based on RHEL.