Process: Process is an instance of an executing program. For example, we write our computer programs in a text file and when we execute this program, it becomes a process which performs all the tasks mentioned in the program.
Program: Program is a set of instructions to perform a certain task. Eg: chrome.exe, notepad.exe
Thread: Thread is a path of execution within a process. A thread is also known as a lightweight process. The idea is to achieve parallelism by dividing a process into multiple threads. For example,Word processor uses multiple threads: one thread to format the text, another thread to process inputs.
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