Swap space represents an area on the physical disk (usually a dedicated partition) which holds temporarily a process memory image. This area is often called virtual memory because it allows processes to use memory beyond the physical RAM available on your computer. Swapping and paging algorithms allow processes or portions of processes to move between physical memory and a mass storage device. This frees up space in physical memory.
As the swap space is stored on the disk the access times tend to be slow compared to the RAM. In Linux we can have two types of swap space: a dedicated partition on the physical disk or a swap file which resides among other files on the filesystem. Linux kernels newer that 2.4.10 allow 32 swap areas.