*RAM disk* is a term from the past when DOS was alive and information was stored on disks instead of internet. If you created image of some disk, it was possible to load it into memory. Memory disks were useful to load software from Live CDs. Usually software needs some space to write data during boot sequence, and RAM is the fastest way to setup one.
Filesystem space in memory can be extremely useful today too. For example, to run tests without reducing resource of SSD. While the idea is not new, there was no incentive to explore it until I've run upon tmpfs reference in Ubuntu Wiki.
For example, to get 2Gb of space for files in RAM, edit /etc/fstab to add the following line:
/var/ramspace is now the place to store your files in memory.
tmpfs /var/ramspace tmpfs defaults,size=2048M 0 0