Meaning "goods of weight" in French, avoirdupois pounds and ounces are the ones we were all taught in elementary school. As mentioned above, one avoirdupois pound converts into 16 avoirdupois ounces. If, for example, you wanted to know the weight in ounces of a 15 lb barbell, you would multiply the number of pounds by 16, giving you 240 ounces.
Probably named after the city Troyes in Champagne, France, famous for its lively markets since medieval times, the Troy ounce is used for determining the price of precious metals like gold, silver and platinum, and well as for gemstones and gunpowder. Whereas the avoirdupois ounce is made of 437.5 grains, the next smaller traditional weight unit, the Troy ounce has 480 grains, making it slightly heavier. Originally, 12 Troy ounces made one Troy pound, but the pound has fallen out of use.
Fluid ounces differ from avoirdupois ounces in that they are measures of volume rather than mass. In the traditional British system, however, they come out to the same thing, with one Imperial fluid ounce being equivalent to one avoirdupois ounce of water. Hence, one avoirdupois pound of water converts to 16 fluid ounces in the Imperial system. Though pounds are not commonly spoken of when referring to liquid, the Imperial gallon is made of 10 avoirdupois pounds of water.
For reasons lost to history, the U.S. fluid ounce is slightly larger than its British equivalent. A U.S. Standard fluid ounce is equivalent to 1.041 avoirdupois ounces of water at 62 degrees Fahrenheit. So a single avoirdupois pound of water would convert to 15.37 fluid ounces.