A program is a set of computer instructions that perform a particular task. That program can be written in assembler, a very low level computer language, or in a high level, machine independent language such as the C programming language.
An operating system is a special program that allows the user to run applications such as spreadsheets and word processors. This chapter introduces basic programming principles and gives an overview of the aims and functions of an operating system.
A process could be thought of as a program in action. Each process is a separate entity that is running a particular program. If you look at the processes on your Linux system, you will see that there are rather a lot of processes running at any given moment.