An excerpt from an article (Christoph Rohland, Nov 2001):

Tmpfs is a file system which keeps all files in virtual memory.

Everything is temporary in the sense that no files will be created on your hard drive. If you reboot, everything in tmpfs will be lost.

In contrast to RAM disks, which get allocated a fixed amount of physical RAM, tmpfs grows and shrinks to accommodate the files it contains and is able to swap unneeded pages out to swap space.