Red Hat Enterprise Linux (RHEL) is a Linux distribution developed by Red Hat and targeted toward the commercial market. It is often abbreviated to RHEL, although this is not an official designation.
Red Hat Enterprise Linux is frequently used to power high security, high availability, and large scale servers across the enterprise and the internet. Red Hat systems are also frequently used as a platform for large database systems like Oracle and ERP systems like Banner.
Red Hat Enterprise Linux is built from open source software, but the Red Hat Enterprise distribution is built with support comments that are licensed with a subscription cost. For a free open source alternative that is identical minus the support, look at CentOS.