First of all I want to appologize to anyone English who is offended by my calling our obsolete system of mesurement the "English" System. The system of measurement many of us in the US are familiar with is indeed slightly different from the English system of measurement, but they are equally as senseless, and most people commonly refer to it as the English system.
The US gallon is a different size to the UK gallon so that NO liquid measures of the same name are the same size in the US and UK systems.
Also that the ton(US) is 2000 pounds while a ton(UK) is 2240 pounds. These are also referred to as a short ton and long ton respectively.
In matters concerned with land measurements, for the most accurate work, it is necessary to establish whether the US survey measures are being used or not.
The measurement system commonly used in the United States today is nearly the same as that brought by the colonists from England. These measures had their origins in a variety of culturesBabylonian, Egyptian, Roman, AngloSaxon, and Norman  French. The ancient "digit," "palm,","span," and "cubit" units evolved into the "inch," "foot," and "yard" through a complicated transformation not yet fully understood.
Roman contributions include the use of the number 12 as a base (our foot is divided into 12 inches) and words from which we derive many of our present measurement unit names. For example, the 12 divisions of the Roman "pes," or foot, were called unciae. Our words "inch" and "ounce" are both derived from that Latin word.
The "yard" as a measure of length can be traced back to the early Saxon kings. They wore a sash or girdle around the waist that could be removed and used as a convenient measuring device. Thus the word "yard" comes from the Saxon word "gird" meaning the circumference of a person's waist.
Standardization of the various units and their combinations into a loosely related system of measurement units sometimes occurred in fascinating ways. Tradition holds that King Henry I decreed that the yard should be the distance from the tip of his nose to the end of his thumb. The length of a furlong (or furrowlong) was established by early Tudor rulers as 220 yards. This led Queen Elizabeth I to declare, in the 16th century, that henceforth the traditional Roman mile of 5,000 feet would be replaced by one of 5,280 feet, making the mile exactly 8 furlongs and providing a convenient relationship between two previously ill related measures.
Thus, through royal edicts, England by the 18th century had achieved a greater degree of standardization than the continental countries. The English units were well suited to commerce and trade because they had been developed and refined to meet commercial needs. Through colonization and dominance of world commerce during the 17th, 18th, and 19th centuries, the English system of measurement units was spread to and established in many parts of the world, including the American colonies.
However, standards still differed to an extent undesirable for commerce among the 13 colonies. The need for greater uniformity led to clauses in the Articles of Confederation (ratified by the original colonies in 1781) and the Constitution of the United States (ratified in 1790) giving power to the Congress to fix uniform standards for weights and measures. Today, standards supplied to all the States by the National Institute of Standards and Technology assure uniformity throughout the country.
Information taken from the City of Brockton Department of Weights and Measures http://www.brocktonmass.com/weights/history.html
If you have tried it yet, there is a simple quiz on the home page that lets you check out some of the basic differences.
This page is designed to answer questions on the origin of this strange system of measurement many people seem to feel comfortable with.
Let's start with a summary of the English system:
Length 
Area 
12 inches = 1 foot 
144 sq. inches = 1 square foot 
3 feet = 1 yard 
9 sq. feet = 1 square yard 
220 yards = 1 furlong 
4840 sq. yards = 1 acre 
8 furlongs = 1 mile 
640 acres = 1 square mile 
5280 feet = 1 mile 
1 sq.mile = 1 section 
1760 yards = 1 mile 
36 sections = 1 township 
Volume 
1728 cu. inches = 1 cubic foot 
27 cu. feet = 1 cubic yard 
Capacity (Dry) 
Capacity (Liquid) 

16 fluid ounces = 1 pint 
2 pints = 1 quart 
4 gills = 1 pint 
8 quarts = 1 peck 
2 pints = 1 quart 
4 pecks = 1 bushel 
4 quarts = 1 gallon (8 pints) 
Mass 
437.5 grains = 1 ounce 
16 ounces = 1 pound (7000 grains) 
14 pounds = 1 stone 
100 pounds = 1 hundredweight [cwt] 
Troy Weights 
24 grains = 1 pennyweight 
20 pennyweights = 1 ounce (480 grains) 
12 ounces = 1 pound (5760 grains) 
20 cwt = 1 ton (2000 pounds) 
Apothecaries' Measures 
Apothecaries' Weights 
60 minims = 1 fl.dram 
20 grains = 1 scruple 
8 fl.drams = 1 fl.ounce 
3 scruples = 1 dram 
16 fl.ounces = 1 pint 
8 drams = 1 ounce (480 grains) 

12 ounces = 1 pound (5760 grains) 
Why?
Now for why some of these measuremnents are what they are.
Mile
The word mile comes from the Latin word mille, which means 1000, as the original mile was defined as 1000 paces of a Roman Legion.
The definition has changed many times over the years, but it eventually (in the year 1592) settled to being defined as 5280 feet. Which is 1760 yards, as there are 3 feet in a yard.
The definition of the mile has not changed since 1592, but the definition of the foot has, so the actual length of the mile has changed a couple times. That is why there is a survey mile and the international mile. The survey mile is 5280 survey feet, and the international mile is 5280 international feet.
more to come