Most of the world uses the Metric system (or SI), with one of the few exceptions being the United States. The SI system of units is logical, simple to understand and international, making it quite confusing why the US hasn't adopted it. Many critics of the US system however, fail to see some very fundamental problems with a switch.
The US Standard system of units is unfortunately both archaic and convoluted. I'm certain that by saying this I'm already angering quite a few Americans (and to be honest I can understand a certain brand loyalty to something as widespread as a system of units), but it is true nonetheless. 12 inches make a foot. Three feet make a yard, and 1760 yards make a mile. There's no consistency or logic behind the scaling and it's utterly unscientific, relying instead on arbitrary standard measurements for, say, how big a human foot should be. An outsider who attempts to learn this system from scratch is going to have a difficult time adjusting. The sheer number of different units (Gizmodo's recent article on this listed 30 different units of volumes, a staggering amount) only adds to the difficulty.
The consistency of the metric system is one of it's appeals
The Metric system however, uses one unit and factors it by ten for scale: The standard unit of length is the meter. A decimeter is a tenth of a meter. A centimeter is one hundredth of a meter. A millimeter is one thousandth of a meter. On the other end of things, a kilometer is one thousand meters. It's simple to understand and consistent: If you know what a meter is, you will very easily realize what a kilometer is. The prefixes permeate the entire system as well – from volumes, such as the milliliter (one thousandth of a liter) to mass, like the kilogram (one thousand grams).
Only a few countries still don't use the metric system
Then there's always the fact that most of the world uses the Metric system (and in the scientific world, the almost identical SI system). This causes practical issues in commerce and trade between the US and the rest of the world. Machined parts for example, do not naturally fit each other, and tool sizes are different. So why is the United States still using this system? Well, if it was as easy as just selecting "metric" in a drop-down menu on some computer in the bowels of the pentagon, they probably would have changed by now. In reality, it's not that simple and there are massive practical issues with switching. To understand this, we need to go back in time, and learn about how this system came to be.