tmpfs is a file system that stores all files in virtual memory (meaning it will also use the swap space). It adjusts the actual memory size it uses to fit the files, so it doesn't used a fixed amount of memory. It allows to specify a maximum size with the size option when (re)mounting. The default size is half the available RAM.
It is mainly used to implement POSIX shared memory in Linux, by mounting a tmpfs file system on /dev/shm. It is also often mounted on /tmp, /var/tmp or some other personal tmp directory to increase performance of these often used directories. It is also used by udev (which mounts a tmpfs on /dev/).
mounting a tmpfs on /tmp with max size of 512 Megabytes and permissions of 1777:
mount -t tmpfs tmpfs /tmp -o size=512M,mode=1777
the equivalent entry in the fstab
none /tmp tmpfs size=512M,mode=1777 0 0
Increase the size
Increase the size of a ramdisk:
mount -t tmpfs tmpfs /tmp -o size=2000M,mode=1777,remount
This increases the ramdisk's size to 2000M. No file is destroyed.