macOS is an operating system designed by Apple. The operating system is what allows you to use a computer. macOS comes preloaded on all new Apple Macintosh computers (commonly known as Macs).
macOS makes it possible to complete all kinds of everyday tasks on your computer. For example, you can use it to browse the Internet, check your email, edit digital photos, listen to music, and play games.
macOS is also used in some offices because it gives you access to productivity tools such as calendars, word processors, and spreadsheets.
Earlier versions of macOS have been around since the mid-1980s. There have been many versions since then, but the most recent ones include macOS Mojave (2018), High Sierra (2017), Sierra (2016). El Capitan (2015), and Yosemite (2014).
In some older versions, macOS was called OS X (pronounced O-S ten). However, many people use the terms OS X and macOS interchangeably because the basic functionality of the operating system is still very similar.