Linux

Virtual memory is different from swap memory.

Swap memory is used when you run out of physical memory, so the memory is extended to your storage.

Virtual memory is an abstraction that lies between programs using memory and the physical memory in the device. It can be something like compression and memory-mapped files, like mentioned.

And yes, some swap is still useful, up to something like 4G for larger systems.

And if you want to hibernate to disk, you may need as much swap as your physical memory. But maybe that’s changed. I haven’t done that in years.

Virtual memory isn't swap, it is a mechanism that allows the operating system to give processes a view of memory that is almost completely decoupled from real physical memory and other processes. For example some programs require their code and data to be placed at exact memory locations in order to work - virtual memory allows you to run as many of these programs as you wish, because one process's address 0x1000 has nothing to do with another one's 0x1000, unless they set it up as shared memory (but even the same chunk of shared memory might be mapped to different addresses in the processes that share it).

Swapping is a cool trick that you can do with virtual memory, though. Basically you store a piece of memory somewhere outside the physical memory, and then make the address invalid in virtual memory. When the process tries to access it, it will crash. The OS will be notified of the crash, see that it was due to the process trying to access swapped out memory, load the chunk back from disk (maybe to a different physical location), update the virtual memory to correctly point to this chunk, and restart the crashed process from the instruction that caused the crash. So from the point of view of the process, nothing went wrong at all, except that one instruction took a very long time to execute.

Also, isn't it harmful to SSDs?

Swapping doesn't do enough writes to matter, unless your system is running really low on RAM.