SMBus (System Management Bus) is an implementation of I2C, a type of serial data bus. SMBus is just I2C at 100kbps (lowest speed of I2C, there are faster variants)) and is a bus designed for hooking up power management and various other system-related stuff. Its used in laptops for connecting 'Smart' batteries, temperature sensors, etc, and most mobos have SMBus headers. Also, Matrix Orbital LCDs can connect to SMBus, which is much more convienant (no wire sticking out of case or ugly USB adapter inside). SMBus isnt without flaws tho, the biggest one being addressing. When it was designed, most laptops had one battery. So it was designed that all batteries would have the same address. This is why many chipsets now have two SMBus headers- because you need one for each battery.
The card would use SMBus for a few reasons... 1.Most chipsets have it, 2.Nothing sticking outside the computer (like a serial cable) is required, and 3.the fan controllers I'm looking at have I2C connections.
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