A FileSystem that lives entirely in the Kernel's filesystem Cache in order to offer greatly accelerated access times to its contents. Unlike a RamDisk it does not simulate a block device, and can therefore grow and shrink as needed to accommodate the files it hosts. Of course, files stored on in-memory FileSystems like RamDisks or a Tmpfs vanish when the system shuts down.
Any partition that will have many, frequently accessed, small files which do not require permanent storage is a good candidate for Tmpfs. Of all the directories in the FileSystemHierarchy, /tmp is the canonical candidate. Because Tmpfs only takes up as much space as needed, putting /tmp on a Tmpfs tends to reduce the memory available to other processes by only a negligible amount on modern machines equipped with large amounts of RAM.