For ease of use in office equipment, file folders and storage cabinets, paper sizes need to use a standardized system so that one sheet of paper will be consistent with the next. There are two prevailing standards in the world. The custom system in North America, which evolved from United States standards, dominates in the U.S., Canada and Mexico. Nearly everywhere else, the ISO 216 international standard dominates.
The individual paper sizes in these two standards differ, so when traveling between these regions or working in settings where both types are likely to be present, it's important to use the correct papers and keep track of which are which. The most common North American paper is the letter size, with imperial dimensions of 8.5 inches by 11 inches. Nearly everywhere else, the most prevalent size of general-purpose paper is the A4, with metric dimensions of 210 millimeters by 297 millimeters (8.27 inches by 11.69 inches). The most common densities of these two sizes of paper also differ; a sheet of the traditional 20-pound, letter-size paper weighs about 72 grams per square meter, while the most common density of A4 paper weighs about 80 grams per square meter.
Most office equipment has settings for both standards, so for example if you need to print a document for a given paper size, simply insert the correct type of paper and specify that type in the printer control prompt. Note that if you use the wrong type of paper, you'll get uneven margins.