A percentage is a number or ratio expressed as a fraction of 100. 45% is a way to represent how many parts out of 100 some object or items that may be are how many you have—it is the simplest way to represent proportions and to compare quantities in a common format. Percentages are not physical quantities, so, unlike true measurements, they are unitless; they simply describe a ratio between two numbers. Other ways to describe these relationships would be through ratios, fractions, and decimals, but in percent form, it is frequently the easiest to see at a glance because it in essence, compares everything back to 100 as the same reference point.