Basicly just a buffer memory of 128mb or so which stores the latest data, or the most often used stuff, decreasing harddisk access by a major amount.
If the buffer is designed to hold on to stuff for longer times, this brings up another potential problem: A crash can undo a lot of work, if the actually data never made it to the harddrive before your computer takes a dive...
Even more so if you don't have a properly journaling filesystem like EXT3 (which is a lot more resilient than FAT or NTFS which windows uses)