Back-UPS: Stand-by (either on AC or on battery)
Back-UPS Pro: Line-Interactive (can boost or trim voltage if it gets a little high or low without using battery)
Smart-UPS: Line-Interactive + Pure sine wave (Not really necessary for computer loads)
I like the Back-UPS Pro series since it gives the best performance for the price. The weird thing is the Back-UPS Pro actually has better voltage regulation than the Smart-UPS.

BTW, that only applies to the older Back-UPS Pro's (I don't know about the newer USB series).