Software (SW) are programs that run on a computer, i.e. its non-physical parts (as opposed to hardware); for example an operating system, the internet browser etc. Software is created by programming.
Usually we can pretty clearly say what is software and what is hardware, but there are cases where it's debatable. Normally software is that about the computer which can relatively easily be changed (i.e. reinstalled by a typing a few commands or clicking a few buttons) while hardware is hard-wired, difficult to modify, and not expected or designed to be modified. Nevertheless e.g. some firmware is kind of software in form of instructions which is however many times installed in some special kind of memory that's difficult to reprogram and not expected to be reprogrammed often -- some software may be "burned in" into a circuit so that it could only be changed by physically rewiring the circuit (the ME spyware in Intel CPUs has a built-in minix operating system). And this is where it may sometimes be difficult to decide where the line is drawn. This issue is encountered e.g. by the FSF which certifies some hardware that works with free software as "Respects Your Privacy" (RYF), and they have very specific definition what to them classifies software.
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