What's the difference?

What's the difference between ATA100, ATA133, ATA150, ect.? And what excatly does each one mean? I've tried looking for it but didn't find anything.

Quite simply, the higher the number the faster the maximum possible read/write speed of the hard drive (subject to the physical limitations of the hardware, of course - it might come from the drive's on-board cache at that rate, but sustained transfers are usually much lower than the theoretical maximum due to physical/mechanical limitations). ATA100 has a theoretical maximum transfer rate of 100Mbps, ATA133 of 133Mbps, etc.