Mainframe computers are often seen as ancient machines—practically dinosaurs. But mainframes, which are purpose-built to process enormous amounts of data, are still extremely relevant today. If they’re dinosaurs, they’re T-Rexes, and desktops and server computers are puny mammals to be trodden underfoot.

It’s estimated that there are 10,000 mainframes in use today. They’re used almost exclusively by the largest companies in the world, including two-thirds of Fortune 500 companies, 45 of the world’s top 50 banks, eight of the top 10 insurers, seven of the top 10 global retailers, and eight of the top 10 telecommunications companies. And most of those mainframes come from IBM.

In this explainer, we’ll look at the IBM mainframe computer—what it is, how it works, and why it’s still going strong after over 50 years.