A program, or software, is a set of coded instructions written for a computer or mobile device, such as an operating system program or an application program
System software is the software that runs a computer, including the operating system and utilities.
An OS is a program that manages the complete operation of your computer or mobile device and lets you interact with it (aka platform)
Utilities enable you to perform maintenance-type tasks related to managing the computer or device, e.g. managing files, search utility, uninstall program, file compression, back-up, etc
Application software are programs that help you perform specific tasks when using your computer or mobile device