Micro drive is a type of drivetrain, mostly BMX and MTB, that uses smaller than standard sized cogs. The smallest rear sprocket that fits on a freehub body is an 11-tooth, but with the use of a cassette hub, sometimes called a micro drive rear hub, sprockets as small as 8 teeth may be used.
The advantage of micro drive is that it means a smaller front sprocket, or chainring, can be used without affecting the gear ratio, providing better ground clearance. For example, a bike using a 32t chainring with a 16t rear sprocket can change to a 18t chainring and a 9t sprocket, keeping the 2:1 ratio. Another advantage is the reduction in weight with the reduction in size of all the parts.
The disadvantage is increased stresses on the drive train, leading to increased wear and even premature failure. Chain stretching and breaking can occur more easily. As the front and rear sprockets become smaller, the drive tension in the chain increases. Designers can overcome these issues by choosing materials with higher yield strengths, however usually at higher costs.