Normally, RAM is used for temporary storage of data. But sometimes RAM data is stored in the hard disk, what is this method called?

Cache memory is a small, fast layer of memory (typically SRAM) placed between the CPU and main RAM to store frequently accessed data - it does not involve the hard disk.

Static memory (SRAM) refers to a type of semiconductor RAM that holds data without needing periodic refresh cycles - it is a hardware characteristic, not a memory management technique.

Virtual memory is a memory management scheme in which the operating system uses a portion of the hard disk - called a page file or swap space - to store RAM contents that do not fit in physical memory, allowing processes to use more memory than is physically installed. The OS transparently moves pages between RAM and disk based on access patterns, making the combined space appear as a single large memory pool.

Volatile memory describes any memory that loses its stored data when power is removed, such as RAM itself - this is a property of a memory type, not a technique for extending storage to disk.