Solid State Drive. Much faster to read/write/search. I do cad @ home off a flash drive... but I'm betting that's not quite the same as the new internal SSD setups.
To add on what ProDarwin said, SSDs use solid-state memory, i.e. flash memory. There are no moving parts.
What worries me is their limited write cycles. CAD files need a lot of rewrites, so I wonder if life expectancy of those drives used for CAD could be really short.
From what I've read in some other forums where these devices have been discussed, perhaps the best utilization would be to put all of the files which tend to be only and not written to all that often, such as the executable programs and files, including standard library parts, which are referenced by other files (such as standard fasteners in an Assembly) but which are not often written to. Granted, this would not necessarily speed-up the opening of large assemblies (except for the standard library parts), but the primary program would certainly start quicker and run faster as modules are opened and swapped in as different applications are launched or utilities are accessed.
Note that I have no direct experience with SSD's, yet, but I have been following this topic on the net and have been trying to keep my ears open for real-world feedback and user experiences, which are still rather limited in number.
John R. Baker, P.E.
Product 'Evangelist'
Product Design Solutions
Siemens PLM Software Inc.
Industry Sector
Cypress, CA