The /proc filesystem is a virtual filesystem, which means that it doesn't really exist except in the "head" of the Linux kernel. The /proc filesystem as described here is specific to the Linux kernel, even though it may very well be present in other operating systems as well, but with a different functionality and a different meaning.
By virtual filesystem, we simply mean that there is no trace of it on top of any of your harddrives, which all filesystems would normally leave. Nothing that you ever do to a virtual filesystem will ever do any changes to the actual harddrives themselves, but only in the primary memory. Virtual filesystems are created "on the fly" by the kernel during bootup, and it is always updated every single time you enter the filesystem, or do anything within it.
Virtual filesystems may take several other incarnations as well. One such is a simple RAM Harddrive, where you will be able to save files while the machine is running. On diskless Amigas this was often used to move files from one floppy to another floppy by first moving the original file to the RAM memory and then swap floppies and copy it to the new floppy. However, these files would disappear as soon as you reset or restarted the machine.