Most disks (more accurately, volumes) are backed by some sort of permanent storage, such as an optical disc (as in CD and DVD), magnetic hard disk, or flash memory. In all of these cases, the data on the volume will persist after the computer is shut down because it's been written to permanent storage. (Sometimes that isn't true, but it's very very rare.)
A RAM disk, on the other hand, has no permanent storage behind it. The data on the RAM is stored only in RAM (memory) and will be forgotten at shutdown.
Because it's in RAM, a RAM disk is very, very fast, in fact, it's the fastest disk you can possibly have (without buying an expensive RAID). On some Macs, you can write data to a RAM disk at more than half a gigabyte per second.
It's great for things that you need to read or write quickly, especially if you don't need to keep them around. A RAM disk is great for iShowU temp files (see the Storage tab of its preferences). You can also put your Downloads folder (in Safari, Mail, Adium, etc.) on a RAM disk since you won't always want to keep things you download.
By default, the app simply creates a 64-MiB RAM disk named "RAM Disk", then exits (making it suitable to put in your Login Items). If you hold down the Option key while launching it, you can change these settings, either for that RAM disk only or until further notice.
Mac OS X 10.4 PPC,
Mac OS X 10.5 PPC,
Mac OS X 10.4 Intel,
Mac OS X 10.5 Intel