You guys are retarded. they're specific reasons as to why. not just because one is 'smarter' he is looking for a detailed answer. I've the same questions. I've found on Wiki Answers this long answer:
The smart UPS notifies the computer when power is switched to battery, it also notifies the computer when the battery is almost discharged and the UPS won't be able to keep going. A smart UPS can allow the computer enough time to shut down gracefully, without losing data.

A smart UPS can be controlled by software from a computer, a UPS smart have a serial port or USB connections to the PC and a microprocessor on board. The ones are not smart provides backup power in the event that AC fails completely is a heavy box that plugs into a AC outlet and provides one or more outlets for the computer and its peripherals, it has an on/off switch, requires no maintenance, and is very simple to install. A Smart UPS can be controlled by software; a Non-Smart UPS cannot be.

A smart UPS has the capability to interface with the computer system which is plugged into it. This is generally done with a serial or USB connection. The smart UPS comes with software which will give the user information on the status of the UPS and batteries and can automatically trigger shutdown sequence whenever it detects a power loss.

Im trying to find a more shorter answer.

wow arent you a fucking retard

because its smarter.