The speed of a SATA connection is quoted in Arabic numerals (e.g. 150 MB/s). The Roman numerals following a SATA level represent the revision number. So SATA I.0 and SATA III.0 are, respectively, the first and third revisions of the standard.
A number of things changed between each revision, but the most obvious change to most users will be the speed at which data is transferred.
SATA revision 1.0
The first revision came in January, 2003. The transfer rate was – in theory – 1.5 Gbit/s, which seems tortoise-like now in its slowness but was fast for the time, although – once the encoding overhead had been taken into account, unencoded data actually moved at 1.2 Gbit/s. Revision 1 SATA did not support NCQ (Native Command Queuing), which when it arrived was a significant SATA performance improvement.
SATA revision 2.0
This revision introduced NCQ. Native Command Queuing allows the hard disk to look at the read and write commands it receives and decide the optimum order in which to process them. For the bulk of personal computing, any improvement is miniscule (though disk wear will be reduced – always a good thing) but in server applications with multiple read/write commands arriving frequently and constantly performance improves considerably. There was also a doubling of transfer rate, to 3 Gbit/s.
SATA revision 3.0
Manufacturers are constantly improving architecture and performance of processors and peripherals, and SATA had to keep pace. The third revision, in 2009, doubled the transfer rate once more, this time to 6 Gbit/s, allowing the newly introduced high performance flash drives to perform at full efficiency. The encoding overhead still applied and the actual unencoded transfer rate was 4.8 Gbit/s.
Revision 3.0 introduced a number of other improvements including better NCQ and support for a 7mm optical drive and a connector that would handle the new, smaller, 1.8inch drives.
There has as yet been no Revision 4.0. However, Revision 3.0 has been updated twice – as Revision 3.1 and Revision 3.2 – to introduce compatibility with drives and drive characteristics that had not previously existed.