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Ram disks are virtual disks that use RAM as super fast storage, with the "side effect" that they are not reboot persistent. Modern computers have a lot of RAM, and sometime one may need a super fast disk storage of a limited size to speed up some disk intensive tasks. On Linux you can mount some ram into a folder, and this is very convenient for transparently using ramfs in places where nobody thought about it before.

My little quick reference:

Mount ramfs to a folder

$ sudo mount -t ramfs /dev/ram0  ~/tmp/ram

Remeber to chown and chmod to specify access permissions

Verify mounted ram disks (to check availability)

$ mount | grep ram

Check ramdisk details

$ sudo tune2fs -l /dev/ram0

And this is how the RAM disk will look like on your Ubuntu desktop once mounted:

References:
Linux Ramdisk mini-HOWTO
Using the RAM disk block device with Linux