SCSI Specifications
SCSI-1

SCSI-1 was the first SCSI standard and was developed in 1986. It included support for asynchronous and synchronous SCSI devices. In a asynchronous data transfer, the data is send in irregular intervals. Start and stop bits signal the beginning and the end of a byte. In a synchronous data transfer the data is synchronized by pulses from the computer. This type of data transfer is faster than asynchronous data transfer.
As already mentioned in the introduction SCSI-1 supports up to seven devices.
The maximum data transfer rate is 5 Mbytes per second. 

SCSI-2

SCSI-2 was a significant improvement. The following features were added to SCSI-1: 

Improved connectors (smaller)  
Fast SCSI-2 : by increasing the SCSI bus clock rate which increased the SCSI data transfer rate to 10Mb/sec.  
Wide SCSI : by doubling the width of the bus (bandwidth) from 8 to 16 bits it is possible to transfer twice as much data in the same time (20Mb/sec). 
SCSI bus Parity Checking  
The combination of Fast SCSI and Wide SCSI (Fast Wide SCSI) allows to transfer with a maximum rate of 20 Mb/sec.  
SCSI-3

The SCSI-3 standard is not yet finalized. However the following features will be added: 

Ultra Wide SCSI : data transfers up to 40 Mb/sec. and backward compatible with SCSI-2 and SCSI-1.  
Fiber Channel support 
IEEE P1394 support (FireWire) 
Serial Storage Architecture (SSA) : faster data rates, more devices, longer cable, but not backward compatible with SCSI-2 or SCSI-1.