What Are They?
Microsoft's Windows and and Apple's ios are the main operating systems but there are also others. An operating system will come preinstaled in any modern computer. Macs have ios and most of the others will have Windows. Handheld consoles such as phones and tablets also have operating systems. Apple consoles will have ios and others will have Google's android.
Operating systems let you communicate with your computer. Without the operating system your computer just consists of code.
Computer Basics: Understanding Operating Systems