Right now, you're probably working on a SATA-based hard drive, with a transfer speed of 150MBps (SATA) or 300MBps (SATA2). Meanwhile, Seagate just demonstrated their tentative SATA3 spec, which reaches speeds of 600Mbps.
However, these numbers are all just theoretical spec. In reality, a combination of hardware and software limitations cause SATA drives run far slower than promised—which is something SATA3 hopes to overcome.
Seagate says SATA3 will be 100% faster than SATA2, while being backwards compatible with the existing SATA infrastructure (cables, etc).
However, there are a few catches: Seagate's SATA3 is not yet a standard that other manufacturers have agreed upon (like Western Digital)—with the exception of AMD, who has pledged to support Seagate's standard in their upcoming chipsets.
Also, Seagate doesn't have an actual hard drive to show you yet. However, they do plan to have a product on the market within the year. [CNET]
UPDATE: Upon a second reading, we think that CNET meant to say "100%" faster instead of "200%" faster.