Virtual memory is a volatile memory temporarily created on the storage device. it is created when RAM is almost empty and the computer is running many programs at once.
The operating system makes part of the storage drive available to use as RAM. Virtual memory is much slower than the main memory because processing power is being taken up by moving data around, rather than just executing instructions.
Swapping/paging is the process the operating system uses to move data between RAM and virtual memory. The OS moves data from processes that are not immediately needed out of the RAM and stores them in virtual memory. It then copies the data back into RAM when the process is needed again.
Using virtual memory slows the computer down because copying to a hard disk takes much longer than reading and writing RAM.