The difference between SATA I, II and III is data transfer speed.
SATA I can transfer data at a maximum of 1.5 Gigabits per second.
SATA II can transfer data at a maximum of 3.0 Gigabits per second.
SATA III can transfer data at a maximum of 6.0 Gigabits per second.
The data cables are interchangeable;
no need to buy 'special' SATA III data transfer cables.
SATA III and SATA II are backwardly compatible with SATA I, so you could use a SATA III drive on a SATA I motherboard. You would, of course, only get transfer speeds at the SATA I level though.
Just bear in mind that to achieve 6.0 Gigabits per second transfer rate, both the motherboard (interface) and the drive must be for SATA III. A SATA III drive on a SATA II motherboard will only transfer 3.0 Gigabits per second.