Decimals are most often used when referring to money. If someone has $3.42, for example, he has three whole dollars and 42/100 of a dollar. Most people find it easier to compare two decimals than two fractions.
A decimal is simply a representation of a fraction. It's easier to say a bag of chips costs $0.99 than it is to say it costs 99/100 of a dollar. The decimal form of a fraction is determined by dividing the numerator, or top number, by the denominator, or bottom number. The fraction 4/5 is 0.80 in decimal form because four divided by five is 0.8.
John Napier, a Scottish mathematician, was the person to suggest using decimals to represent fractions. He suggested that numbers to the left of the decimal point should represent whole numbers and numbers to the right of the decimal point should represent fractional numbers.
Decimals are used in many places. At the gas pump, decimals are used to show how much gas is pumped as well as how much the gas costs. If gas is $3.29 per gallon, someone who pumps 15 gallons owes $49.35. This means he owes 49 whole dollars and 35/100 of another dollar. The odometer of a vehicle also uses decimals to keep track of the mileage.