I had never heard of DOT 5 until watching MotorWeek (episode 3425) yesterday. It is apparently a new brake fluid, which unlike other types, does not attract moisture. That means that the brake components can last longer, and the fluid can remain in the system longer.
Important to know though is that it apparently will not work with ABS brake systems as it will bubble. This means that can likely only be used in older vehicles (a plus for me) or in hydraulic clutch systems. My understanding is that it can be used in any system requiring DOT 3 or DOT 4, but again, not in ABS. It is also considerably more expensive, but if it lasts longer, over the long run it may end up cheaper. Oh yeah, and it is PURPLE!
Here is some good info: