If you have not heard of them they are the same as hard disk computers but instead use flash memory instead. I have seen a few laptops being touted by the major manufacturers and a few desktop linux based computers. The main advantages with flash drives is that there are no moving parts and so use less power (less heat) and boot a lot faster. The down side is that flash memory is still expensive compared to hard disks. I couldn't imagine a windows based PC being launched anytime soon but a Linux PC could be diy'ed yourself. Anyone had a go.
A hard disk has moving parts whilst flash has none. Flash is also a lot smaller. Think of flash memory as the same sort of memory card you would use in a digital camera. Similar to ther memory in USB memory sticks.
Good question. I have never used a flash based computer. I have used USB flash drives (memory sticks) and they can be slow with very large amounts of data. Then again hard disks can be slow. I really don't know, maybe someone will wade in soon with some experience.