An enterprise Linux subscription is a paid service that gives a company a tested, supported version of Linux for its servers. It brings updates, security fixes, help when something breaks, and a promise that other major software works on it. Linux itself is free, and anyone can download it. Banks and large companies pay for the subscription, not for the code. Red Hat Enterprise Linux and Ubuntu Pro are well-known examples.
Linux is an operating system, the program that runs a computer and lets other programs use its processor, memory, disks and network. Most of the world's servers and cloud machines run some version of Linux. A version that a group packages, tests and releases is called a distribution. Some distributions are free and maintained by volunteers. Others come from a company that sells subscriptions for them.
What does a large company get for its money? Four things. The first is tested updates. Every change is checked so that it does not break running systems, and each release is supported for about ten years. So a bank does not have to rebuild its servers every year. The second is security fixes. When a serious flaw is found in Linux, it is given a CVE number, and the seller ships a fixed version quickly, often within days. The third is support: engineers to call when a server fails at 2 a.m. The fourth is certification. Makers of databases, banking software and servers test their products on that version, and promise that they will work. Many software makers support their product only if it runs on a certified version. The page on the open source business model explains why companies pay for something free.
Take, for example, a large bank in Mumbai with 3,000 Linux servers. One morning, a serious flaw is announced in a widely used part of Linux. Attackers could use it to take control of a server. By the afternoon, the seller has published a fixed package, tested on the same version that the bank runs. The bank's team tests the fix on 50 servers that night. It installs it on the rest over three nights, using configuration management tools, which apply the same change to many servers at once. One database server fails to restart after the update. The team calls the seller's support line at 2 a.m., and a support engineer finds the cause within an hour. Without a subscription, the bank's own engineers would have had to build and test the fix themselves.
Linux runs directly on servers, or inside each virtual machine created by a hypervisor. Above it run the Java runtime and the Kubernetes platform. A developer who rents a small virtual private server usually picks a free distribution instead.
How is it different from free Linux? The code is almost the same. The difference is the promise: tested releases, a long support period, certification, and a company that answers the phone.