An SSD is a solid state drive as apposed to a HDD, hard disk drive; faster and less prone to damage.
From WiKi "SSDs are distinguished from traditional hard disk drives (HDDs), which are electromechanical devices containing spinning disks and movable read/write heads. SSDs, in contrast, use microchips which retain data in non-volatile memory chips[1] and contain no moving parts.[1] Compared to electromechanical HDDs, SSDs are typically less susceptible to physical shock, quieter, and have lower access time and latency. "
PC World has a good video on the in and outs of SSD's.
Cheers